Develop Reference

Custom Scroll Viewer horizontal bar is in a vertical orientation

I have a custom scrollViewer and the horizontal scroller is showing in a vertical orientation
Here is the code for the Horizontal ScrollBarLine:
<ControlTemplate x:Key="HorizontalScrollBar" TargetType="{x:Type ScrollBar}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition MaxWidth="0" />
<ColumnDefinition Width="0.00001*" />
<ColumnDefinition MaxWidth="0" />
</Grid.ColumnDefinitions>
<Border
Grid.ColumnSpan="3"
Background="#F0F0F0"
CornerRadius="2" />
<RepeatButton
Grid.Column="0"
Width="18"
Command="ScrollBar.LineLeftCommand"
Content="M 4 0 L 4 8 L 0 4 Z"
Style="{StaticResource ScrollBarLineButton}" />
<Track
Name="PART_Track"
Grid.Column="1"
IsDirectionReversed="False">
<Track.DecreaseRepeatButton>
<RepeatButton Command="ScrollBar.PageLeftCommand" Style="{StaticResource ScrollBarPageButton}" />
</Track.DecreaseRepeatButton>
<Track.Thumb>
<Thumb
Margin="0,1,0,1"
Background="{StaticResource NormalBrush}"
BorderBrush="{StaticResource NormalBorderBrush}"
Style="{StaticResource ScrollBarThumb}" />
</Track.Thumb>
<Track.IncreaseRepeatButton>
<RepeatButton Command="ScrollBar.PageRightCommand" Style="{StaticResource ScrollBarPageButton}" />
</Track.IncreaseRepeatButton>
</Track>
<RepeatButton
Grid.Column="3"
Width="18"
Command="ScrollBar.LineRightCommand"
Content="M 0 0 L 4 4 L 0 8 Z"
Style="{StaticResource ScrollBarLineButton}" />
</Grid>
</ControlTemplate>
Here is the full code for the custom ScrollViewer
<Window.Resources>
<SolidColorBrush x:Key="StandardBorderBrush" Color="#888" />
<SolidColorBrush x:Key="StandardBackgroundBrush" Color="LightGray" />
<SolidColorBrush x:Key="HoverBorderBrush" Color="#DDD" />
<SolidColorBrush x:Key="SelectedBackgroundBrush" Color="LightGray" />
<SolidColorBrush x:Key="SelectedForegroundBrush" Color="White" />
<SolidColorBrush x:Key="DisabledForegroundBrush" Color="#888" />
<SolidColorBrush x:Key="GlyphBrush" Color="#444" />
<SolidColorBrush x:Key="NormalBrush" Color="#888" />
<SolidColorBrush x:Key="NormalBorderBrush" Color="#888" />
<SolidColorBrush x:Key="HorizontalNormalBrush" Color="#FF686868" />
<SolidColorBrush x:Key="HorizontalNormalBorderBrush" Color="#888" />
<LinearGradientBrush x:Key="ListBoxBackgroundBrush" StartPoint="0,0" EndPoint="1,0.001">
<GradientBrush.GradientStops>
<GradientStopCollection>
<GradientStop Offset="0.0" Color="White" />
<GradientStop Offset="0.6" Color="White" />
<GradientStop Offset="1.2" Color="#DDDDDD" />
</GradientStopCollection>
</GradientBrush.GradientStops>
</LinearGradientBrush>
<LinearGradientBrush x:Key="StandardBrush" StartPoint="0,0" EndPoint="0,1">
<GradientBrush.GradientStops>
<GradientStopCollection>
<GradientStop Offset="0.0" Color="#FFF" />
<GradientStop Offset="1.0" Color="#CCC" />
</GradientStopCollection>
</GradientBrush.GradientStops>
</LinearGradientBrush>
<LinearGradientBrush x:Key="PressedBrush" StartPoint="0,0" EndPoint="0,1">
<GradientBrush.GradientStops>
<GradientStopCollection>
<GradientStop Offset="0.0" Color="#BBB" />
<GradientStop Offset="0.1" Color="#EEE" />
<GradientStop Offset="0.9" Color="#EEE" />
<GradientStop Offset="1.0" Color="#FFF" />
</GradientStopCollection>
</GradientBrush.GradientStops>
</LinearGradientBrush>
<Style x:Key="ScrollBarLineButton" TargetType="{x:Type RepeatButton}">
<Setter Property="Visibility" Value="Hidden" />
<Setter Property="SnapsToDevicePixels" Value="True" />
<Setter Property="OverridesDefaultStyle" Value="true" />
<Setter Property="Focusable" Value="false" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type RepeatButton}">
<Border
Name="Border"
Margin="1"
Background="{StaticResource NormalBrush}"
BorderBrush="{StaticResource NormalBorderBrush}"
BorderThickness="1"
CornerRadius="2">
<Path
HorizontalAlignment="Center"
VerticalAlignment="Center"
Data="{Binding Path=Content, RelativeSource={RelativeSource TemplatedParent}}"
Fill="{StaticResource GlyphBrush}" />
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsPressed" Value="true">
<Setter TargetName="Border" Property="Background" Value="{StaticResource PressedBrush}" />
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Foreground" Value="{StaticResource DisabledForegroundBrush}" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="ScrollBarPageButton" TargetType="{x:Type RepeatButton}">
<Setter Property="Visibility" Value="Hidden" />
<Setter Property="SnapsToDevicePixels" Value="True" />
<Setter Property="OverridesDefaultStyle" Value="true" />
<Setter Property="IsTabStop" Value="false" />
<Setter Property="Focusable" Value="false" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type RepeatButton}">
<Border Background="Black" />
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="ScrollBarThumb" TargetType="{x:Type Thumb}">
<Setter Property="SnapsToDevicePixels" Value="True" />
<Setter Property="OverridesDefaultStyle" Value="true" />
<Setter Property="IsTabStop" Value="false" />
<Setter Property="Focusable" Value="false" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Thumb}">
<Border
Width="3"
Margin="8,0,-2,0"
Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="0"
CornerRadius="2" />
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<ControlTemplate x:Key="VerticalScrollBar" TargetType="{x:Type ScrollBar}">
<Grid>
<Grid.RowDefinitions>
<RowDefinition MaxHeight="0" />
<RowDefinition Height="0.00001*" />
<RowDefinition MaxHeight="0" />
</Grid.RowDefinitions>
<Border
Grid.RowSpan="3"
Background="Transparent"
CornerRadius="2" />
<RepeatButton
Grid.Row="0"
Height="18"
Command="ScrollBar.LineUpCommand"
Content="M 0 4 L 8 4 L 4 0 Z"
Style="{StaticResource ScrollBarLineButton}" />
<Track
Name="PART_Track"
Grid.Row="1"
IsDirectionReversed="true">
<Track.DecreaseRepeatButton>
<RepeatButton Command="ScrollBar.PageUpCommand" Style="{StaticResource ScrollBarPageButton}" />
</Track.DecreaseRepeatButton>
<Track.Thumb>
<Thumb
Margin="1,0,1,0"
Background="{StaticResource HorizontalNormalBrush}"
BorderBrush="{StaticResource HorizontalNormalBorderBrush}"
Style="{StaticResource ScrollBarThumb}" />
</Track.Thumb>
<Track.IncreaseRepeatButton>
<RepeatButton Command="ScrollBar.PageDownCommand" Style="{StaticResource ScrollBarPageButton}" />
</Track.IncreaseRepeatButton>
</Track>
<RepeatButton
Grid.Row="3"
Height="18"
Command="ScrollBar.LineDownCommand"
Content="M 0 0 L 4 4 L 8 0 Z"
Style="{StaticResource ScrollBarLineButton}" />
</Grid>
</ControlTemplate>
<ControlTemplate x:Key="HorizontalScrollBar" TargetType="{x:Type ScrollBar}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition MaxWidth="0" />
<ColumnDefinition Width="0.00001*" />
<ColumnDefinition MaxWidth="0" />
</Grid.ColumnDefinitions>
<Border
Grid.ColumnSpan="3"
Background="#F0F0F0"
CornerRadius="2" />
<RepeatButton
Grid.Column="0"
Width="18"
Command="ScrollBar.LineLeftCommand"
Content="M 4 0 L 4 8 L 0 4 Z"
Style="{StaticResource ScrollBarLineButton}" />
<Track
Name="PART_Track"
Grid.Column="1"
IsDirectionReversed="False">
<Track.DecreaseRepeatButton>
<RepeatButton Command="ScrollBar.PageLeftCommand" Style="{StaticResource ScrollBarPageButton}" />
</Track.DecreaseRepeatButton>
<Track.Thumb>
<Thumb
Margin="0,1,0,1"
Background="{StaticResource NormalBrush}"
BorderBrush="{StaticResource NormalBorderBrush}"
Style="{StaticResource ScrollBarThumb}" />
</Track.Thumb>
<Track.IncreaseRepeatButton>
<RepeatButton Command="ScrollBar.PageRightCommand" Style="{StaticResource ScrollBarPageButton}" />
</Track.IncreaseRepeatButton>
</Track>
<RepeatButton
Grid.Column="3"
Width="18"
Command="ScrollBar.LineRightCommand"
Content="M 0 0 L 4 4 L 0 8 Z"
Style="{StaticResource ScrollBarLineButton}" />
</Grid>
</ControlTemplate>
<Style x:Key="{x:Type ScrollBar}" TargetType="{x:Type ScrollBar}">
<Setter Property="SnapsToDevicePixels" Value="True" />
<Setter Property="OverridesDefaultStyle" Value="true" />
<Style.Triggers>
<Trigger Property="Orientation" Value="Horizontal">
<Setter Property="Width" Value="Auto" />
<Setter Property="Height" Value="18" />
<Setter Property="Template" Value="{StaticResource HorizontalScrollBar}" />
</Trigger>
<Trigger Property="Orientation" Value="Vertical">
<Setter Property="Width" Value="18" />
<Setter Property="Height" Value="Auto" />
<Setter Property="Template" Value="{StaticResource VerticalScrollBar}" />
</Trigger>
</Style.Triggers>
</Style>
<Style x:Key="FavsScrollViewer" TargetType="{x:Type ScrollViewer}">
<Setter Property="OverridesDefaultStyle" Value="True" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ScrollViewer}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" />
<ColumnDefinition />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<ScrollContentPresenter Grid.Column="1" />
<ScrollBar
Name="PART_VerticalScrollBar"
Maximum="{TemplateBinding ScrollableHeight}"
ViewportSize="{TemplateBinding ViewportHeight}"
Visibility="{TemplateBinding ComputedVerticalScrollBarVisibility}"
Value="{TemplateBinding VerticalOffset}" />
<ScrollBar
Name="PART_HorizontalScrollBar"
Grid.Row="1"
Grid.Column="1"
Maximum="{TemplateBinding ScrollableWidth}"
Orientation="Horizontal"
ViewportSize="{TemplateBinding ViewportWidth}"
Visibility="{TemplateBinding ComputedHorizontalScrollBarVisibility}"
Value="{TemplateBinding HorizontalOffset}" />
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Window.Resources>

You have to define a separate style for each Thumb horizontal and vertical accordingly:
<Style x:Key="ScrollBarThumb" TargetType="{x:Type Thumb}">
<Setter Property="SnapsToDevicePixels" Value="True" />
<Setter Property="OverridesDefaultStyle" Value="true" />
<Setter Property="IsTabStop" Value="false" />
<Setter Property="Focusable" Value="false" />
<Style.Triggers>
<Trigger Property="Name" Value="ThumbH">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Thumb}">
<Border
Height="5"
Margin="8,0,-2,0"
Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="0"
CornerRadius="2" />
</ControlTemplate>
</Setter.Value>
</Setter>
</Trigger>
<Trigger Property="Name" Value="ThumbV">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Thumb}">
<Border
Width="3"
Margin="8,0,-2,0"
Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="0"
CornerRadius="2" />
</ControlTemplate>
</Setter.Value>
</Setter>
</Trigger>
</Style.Triggers>
</Style>
And since I have used the property Name as a style triggering condition it should be set accordingly:
<ControlTemplate x:Key="VerticalScrollBar" TargetType="{x:Type ScrollBar}">
<Grid>
<Grid.RowDefinitions>
<RowDefinition MaxHeight="0" />
<RowDefinition Height="0.00001*" />
<RowDefinition MaxHeight="0" />
</Grid.RowDefinitions>
<Border
Grid.RowSpan="3"
Background="Transparent"
CornerRadius="2" />
<RepeatButton
Grid.Row="0"
Height="18"
Command="ScrollBar.LineUpCommand"
Content="M 0 4 L 8 4 L 4 0 Z"
Style="{StaticResource ScrollBarLineButton}" />
<Track
Name="PART_Track"
Grid.Row="1"
IsDirectionReversed="true">
<Track.DecreaseRepeatButton>
<RepeatButton Command="ScrollBar.PageUpCommand" Style="{StaticResource ScrollBarPageButton}" />
</Track.DecreaseRepeatButton>
<Track.Thumb>
<!--Here Added x:Name-->
<Thumb
x:Name="ThumbV"
Margin="1,0,1,0"
Background="{StaticResource HorizontalNormalBrush}"
BorderBrush="{StaticResource HorizontalNormalBorderBrush}"
Style="{StaticResource ScrollBarThumb}" />
</Track.Thumb>
<Track.IncreaseRepeatButton>
<RepeatButton Command="ScrollBar.PageDownCommand" Style="{StaticResource ScrollBarPageButton}" />
</Track.IncreaseRepeatButton>
</Track>
<RepeatButton
Grid.Row="3"
Height="18"
Command="ScrollBar.LineDownCommand"
Content="M 0 0 L 4 4 L 8 0 Z"
Style="{StaticResource ScrollBarLineButton}" />
</Grid>
</ControlTemplate>
<ControlTemplate x:Key="HorizontalScrollBar" TargetType="{x:Type ScrollBar}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition MaxWidth="0" />
<ColumnDefinition Width="0.00001*" />
<ColumnDefinition MaxWidth="0" />
</Grid.ColumnDefinitions>
<Border
Grid.ColumnSpan="3"
Background="#F0F0F0"
CornerRadius="2" />
<RepeatButton
Grid.Column="0"
Width="18"
Command="ScrollBar.LineLeftCommand"
Content="M 4 0 L 4 8 L 0 4 Z"
Style="{StaticResource ScrollBarLineButton}" />
<Track
Name="PART_Track"
Grid.Column="1"
IsDirectionReversed="False">
<Track.DecreaseRepeatButton>
<RepeatButton Command="ScrollBar.PageLeftCommand" Style="{StaticResource ScrollBarPageButton}" />
</Track.DecreaseRepeatButton>
<Track.Thumb>
<!--Here Added x:Name-->
<Thumb
x:Name="ThumbH"
Margin="0,1,0,1"
Background="{StaticResource NormalBrush}"
BorderBrush="{StaticResource NormalBorderBrush}"
Style="{StaticResource ScrollBarThumb}" />
</Track.Thumb>
<Track.IncreaseRepeatButton>
<RepeatButton Command="ScrollBar.PageRightCommand" Style="{StaticResource ScrollBarPageButton}" />
</Track.IncreaseRepeatButton>
</Track>
<RepeatButton
Grid.Column="3"
Width="18"
Command="ScrollBar.LineRightCommand"
Content="M 0 0 L 4 4 L 0 8 Z"
Style="{StaticResource ScrollBarLineButton}" />
</Grid>
</ControlTemplate>
Note: You can change the Height property of the horizontal thumb and the Width property of the vertical one to fit your needs.

WPF Styled Scrollviewer Not Scrolling

I'm currently trying to implement my own stylized ComboBox in WPF. Everything is working fine, except for the scrollviewer. Simply put, the vertical scroll bar does nothing. Here is an image:
As you can see, the vertical scrollbar is scrolled all the way down, but it did not seem to trigger the effect of actually scrolling. I have 12 dummy items in the Combobox. I was reading online that the ScrollViewer style requires three things to work: ScrollContentPresenter, VerticalScrollBar, and HorizontalScrollBar. I have all three declared in my scrollviewer style, which is as follows:
<Style x:Key="VoidwalkerDarkScrollviewer" TargetType="{x:Type ScrollViewer}">
<Setter Property="HorizontalContentAlignment" Value="Left" />
<Setter Property="VerticalContentAlignment" Value="Top" />
<Setter Property="VerticalScrollBarVisibility" Value="Visible" />
<Setter Property="Padding" Value="4" />
<Setter Property="BorderThickness" Value="1" />
<Setter Property="BorderBrush">
<Setter.Value>
<LinearGradientBrush StartPoint="0.5,0" EndPoint="0.5,1">
<GradientStop Offset="0" Color="#FFA3AEB9" />
<GradientStop Offset="0.375" Color="#FF8399A9" />
<GradientStop Offset="0.375" Color="#FF718597" />
<GradientStop Offset="1" Color="#FF617584" />
</LinearGradientBrush>
</Setter.Value>
</Setter>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ScrollViewer">
<Border
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
CornerRadius="2">
<Grid Background="{TemplateBinding Background}">
<Grid.RowDefinitions>
<RowDefinition Height="*" />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>
<ScrollContentPresenter
x:Name="ScrollContentPresenter"
Margin="{TemplateBinding Padding}"
ContentTemplate="{TemplateBinding ContentTemplate}"
Cursor="{TemplateBinding Cursor}" />
<Rectangle
Grid.Row="1"
Grid.Column="1"
Fill="#FFE9EEF4" />
<ScrollBar
x:Name="VerticalScrollBar"
Grid.Row="0"
Grid.Column="1"
Width="18"
Margin="0,-1,-1,-1"
Background="Red"
IsTabStop="False"
Maximum="{TemplateBinding ScrollableHeight}"
Minimum="0"
Orientation="Vertical"
ViewportSize="{TemplateBinding ViewportHeight}"
Visibility="{TemplateBinding ComputedVerticalScrollBarVisibility}"
Value="{TemplateBinding VerticalOffset}" />
<ScrollBar
x:Name="HorizontalScrollBar"
Grid.Row="1"
Grid.Column="0"
Height="18"
Margin="-1,0,-1,-1"
IsTabStop="False"
Maximum="{TemplateBinding ScrollableWidth}"
Minimum="0"
Orientation="Horizontal"
ViewportSize="{TemplateBinding ViewportWidth}"
Visibility="{TemplateBinding ComputedHorizontalScrollBarVisibility}"
Value="{TemplateBinding HorizontalOffset}" />
</Grid>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
And here is my Combobox style (Omitting extras which are not relevant)
<Style x:Key="VoidwalkerDarkComboBox" TargetType="{x:Type ComboBox}">
<Setter Property="SnapsToDevicePixels" Value="true" />
<Setter Property="OverridesDefaultStyle" Value="true" />
<Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto" />
<Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto" />
<Setter Property="ScrollViewer.CanContentScroll" Value="true" />
<Setter Property="MinWidth" Value="120" />
<Setter Property="MinHeight" Value="20" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ComboBox">
<Grid>
<ToggleButton
Name="ToggleButton"
Grid.Column="2"
ClickMode="Press"
Focusable="false"
IsChecked="{Binding Path=IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
Template="{StaticResource ComboBoxToggleButton}" />
<ContentPresenter
Name="ContentSite"
Margin="3,3,23,3"
HorizontalAlignment="Left"
VerticalAlignment="Center"
Content="{TemplateBinding SelectionBoxItem}"
ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}"
IsHitTestVisible="False" />
<TextBox
x:Name="PART_EditableTextBox"
Margin="3,3,23,3"
HorizontalAlignment="Left"
VerticalAlignment="Center"
Background="Transparent"
Focusable="True"
Foreground="#d0d0d0"
IsReadOnly="{TemplateBinding IsReadOnly}"
Style="{x:Null}"
Template="{StaticResource ComboBoxTextBox}"
Text="{TemplateBinding Text}"
Visibility="Visible" />
<Popup
Name="Popup"
AllowsTransparency="True"
Focusable="False"
IsOpen="{TemplateBinding IsDropDownOpen}"
Placement="Bottom"
PopupAnimation="Slide">
<Grid
Name="DropDown"
MinWidth="{TemplateBinding ActualWidth}"
MaxHeight="{TemplateBinding MaxDropDownHeight}"
SnapsToDevicePixels="True">
<Border
x:Name="DropDownBorder"
Background="#1b1b1c"
BorderBrush="#3f3f46"
BorderThickness="1" />
<ScrollViewer
Margin="4,6,4,6"
SnapsToDevicePixels="True"
Style="{DynamicResource VoidwalkerDarkScrollviewer}"><!-- IF I REMOVE THIS STYLE EVERYTHING WORKS FINE !-->
<StackPanel IsItemsHost="True" KeyboardNavigation.DirectionalNavigation="Contained" />
</ScrollViewer>
</Grid>
</Popup>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="HasItems" Value="false">
<Setter TargetName="DropDownBorder" Property="MinHeight" Value="95" />
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Foreground" Value="LightGray" />
</Trigger>
<Trigger Property="IsGrouping" Value="true">
<Setter Property="ScrollViewer.CanContentScroll" Value="false" />
</Trigger>
<Trigger SourceName="Popup" Property="Popup.AllowsTransparency" Value="true">
<Setter TargetName="DropDownBorder" Property="CornerRadius" Value="0,0,4,4" />
<Setter TargetName="DropDownBorder" Property="Margin" Value="0" />
</Trigger>
<Trigger Property="IsEditable" Value="true">
<Setter Property="IsTabStop" Value="false" />
<Setter TargetName="PART_EditableTextBox" Property="Visibility" Value="Visible" />
<Setter TargetName="ContentSite" Property="Visibility" Value="Hidden" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
As the comment in the style suggests, everything works fine if I simply remove the ScrollViewer style from the combobox...but I need it to complete my dark theme.
Does anyone know what I am missing to make this work?

rewriting ScrollViewer template to change ScrollBar color is an overkill.
ScrollBar is a part of ScrollViewer which is a part of ComboBox. Add a default ScrollBar style in ComboBox style Resources and change color via setter:
<Style TargetType="ComboBox">
<Style.Resources>
<Style TargetType="ScrollBar">
<Setter Property="Background" Value="Red"/>
</Style>
</Style.Resources>
</Style>

I figured it out. Just needed to dig a little deeper online. Here's some working XAML(Some of it may not be needed, but it's for full combobox styling) and a picture to show it)
<ControlTemplate x:Key="ComboBoxToggleButton" TargetType="ToggleButton">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition />
<ColumnDefinition Width="20" />
</Grid.ColumnDefinitions>
<Border
x:Name="Border"
Grid.ColumnSpan="2"
Background="#3f3f46"
BorderBrush="#3f3f46"
BorderThickness="1" />
<Border
Grid.Column="0"
Margin="1"
Background="#333337"
BorderBrush="#3f3f46"
BorderThickness="0,0,1,0" />
<Path
x:Name="Arrow"
Grid.Column="1"
HorizontalAlignment="Center"
VerticalAlignment="Center"
Data="M 0 0 L 4 4 L 8 0 Z"
Fill="#d0d0d0" />
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsEnabled" Value="False">
<Setter TargetName="Border" Property="Background" Value="LightGray" />
<Setter TargetName="Border" Property="BorderBrush" Value="Gray" />
<Setter Property="Foreground" Value="DarkGray" />
<Setter TargetName="Arrow" Property="Fill" Value="DarkGray" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
<ControlTemplate x:Key="ComboBoxTextBox" TargetType="{x:Type TextBox}">
<ScrollViewer
x:Name="PART_ContentHost"
Background="{TemplateBinding Background}"
Focusable="False" />
</ControlTemplate>
<ControlTemplate x:Key="ScrollBackground" TargetType="RepeatButton">
<Border Background="Transparent" />
</ControlTemplate>
<Style x:Key="ScrollThumbStyle" TargetType="Thumb">
<Setter Property="SnapsToDevicePixels" Value="True" />
<Setter Property="OverridesDefaultStyle" Value="true" />
<Setter Property="IsTabStop" Value="false" />
<Setter Property="Focusable" Value="false" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Thumb">
<Rectangle
Width="13"
Fill="#7D7D7D"
RadiusX="5"
RadiusY="5" />
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<!-- ComboBox style -->
<Style x:Key="VoidwalkerDarkComboBox" TargetType="{x:Type ComboBox}">
<Style.Resources>
<Style TargetType="ScrollBar">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ScrollBar">
<Grid Width="15">
<Border
Width="13"
HorizontalAlignment="Center"
Background="#33555555"
CornerRadius="5" />
<Track
Name="PART_Track"
Width="{TemplateBinding Width}"
HorizontalAlignment="Center"
IsDirectionReversed="true"
Maximum="{TemplateBinding Maximum}"
Minimum="{TemplateBinding Minimum}"
Value="{TemplateBinding Value}">
<Track.DecreaseRepeatButton>
<RepeatButton Command="ScrollBar.LineUpCommand" Template="{StaticResource ScrollBackground}" />
</Track.DecreaseRepeatButton>
<Track.IncreaseRepeatButton>
<RepeatButton Command="ScrollBar.LineDownCommand" Template="{StaticResource ScrollBackground}" />
</Track.IncreaseRepeatButton>
<Track.Thumb>
<Thumb Style="{StaticResource ScrollThumbStyle}" />
</Track.Thumb>
</Track>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Style.Resources>
<Setter Property="SnapsToDevicePixels" Value="true" />
<Setter Property="OverridesDefaultStyle" Value="true" />
<Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto" />
<Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto" />
<Setter Property="ScrollViewer.CanContentScroll" Value="true" />
<Setter Property="MinWidth" Value="120" />
<Setter Property="MinHeight" Value="20" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ComboBox">
<Grid>
<ToggleButton
Name="ToggleButton"
Grid.Column="2"
ClickMode="Press"
Focusable="false"
IsChecked="{Binding Path=IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
Template="{StaticResource ComboBoxToggleButton}" />
<ContentPresenter
Name="ContentSite"
Margin="3,3,23,3"
HorizontalAlignment="Left"
VerticalAlignment="Center"
Content="{TemplateBinding SelectionBoxItem}"
ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}"
IsHitTestVisible="False" />
<TextBox
x:Name="PART_EditableTextBox"
Margin="3,3,23,3"
HorizontalAlignment="Left"
VerticalAlignment="Center"
Background="Transparent"
Focusable="True"
Foreground="#d0d0d0"
IsReadOnly="{TemplateBinding IsReadOnly}"
Style="{x:Null}"
Template="{StaticResource ComboBoxTextBox}"
Text="{TemplateBinding Text}"
Visibility="Visible" />
<Popup
Name="Popup"
AllowsTransparency="True"
Focusable="False"
IsOpen="{TemplateBinding IsDropDownOpen}"
Placement="Bottom"
PopupAnimation="Slide">
<Grid
Name="DropDown"
MinWidth="{TemplateBinding ActualWidth}"
MaxHeight="{TemplateBinding MaxDropDownHeight}"
SnapsToDevicePixels="True">
<Border
x:Name="DropDownBorder"
Background="#1b1b1c"
BorderBrush="#3f3f46"
BorderThickness="1" />
<ScrollViewer Margin="4,6,4,6" SnapsToDevicePixels="True">
<StackPanel IsItemsHost="True" KeyboardNavigation.DirectionalNavigation="Contained" />
</ScrollViewer>
</Grid>
</Popup>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="HasItems" Value="false">
<Setter TargetName="DropDownBorder" Property="MinHeight" Value="95" />
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Foreground" Value="LightGray" />
</Trigger>
<Trigger Property="IsGrouping" Value="true">
<Setter Property="ScrollViewer.CanContentScroll" Value="false" />
</Trigger>
<Trigger SourceName="Popup" Property="Popup.AllowsTransparency" Value="true">
<Setter TargetName="DropDownBorder" Property="CornerRadius" Value="0,0,4,4" />
<Setter TargetName="DropDownBorder" Property="Margin" Value="0" />
</Trigger>
<Trigger Property="IsEditable" Value="true">
<Setter Property="IsTabStop" Value="false" />
<Setter TargetName="PART_EditableTextBox" Property="Visibility" Value="Visible" />
<Setter TargetName="ContentSite" Property="Visibility" Value="Hidden" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
My best guess is that originally overriding the ScrollViewer itself must have broke something internally...perhaps I left something out. I don't know. Either way, a solution/alternative is just to provide a style for the scrollbars themselves, and ignore the ScrollViewer altogether.

Add a scrollviewer to a contextmenu

I want to add a scrollviewer into my contextMenu because I can have a lot (30-40) of subitems.
So I added the scrollviewer in the submenuHeader Template :
<ControlTemplate
x:Key="{x:Static MenuItem.SubmenuHeaderTemplateKey}"
TargetType="MenuItem">
<Border Name="Border"
BorderBrush="Pink"
Background="Transparent"
Cursor="Hand">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" SharedSizeGroup="Icon"/>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="Auto" SharedSizeGroup="Shortcut"/>
<ColumnDefinition Width="13"/>
</Grid.ColumnDefinitions>
<ContentPresenter
Name="Icon"
Margin="6,0,6,0"
VerticalAlignment="Center"
ContentSource="Icon"/>
<ContentPresenter
Name="HeaderHost"
Grid.Column="1"
ContentSource="Header"
RecognizesAccessKey="True"/>
<TextBlock x:Name="InputGestureText"
Grid.Column="2"
Text="{TemplateBinding InputGestureText}"
Foreground="{StaticResource CouleurTexteBouton}"
Margin="5,2,2,2"
DockPanel.Dock="Right"/>
<Path
Grid.Column="3"
Name="CheckMark"
HorizontalAlignment="Center"
VerticalAlignment="Center"
Data="M 0 0 L 0 7 L 4 3.5 Z"
Fill="{StaticResource CouleurTexteBouton}" />
<Popup
Name="Popup"
Placement="Right"
HorizontalOffset="10"
IsOpen="{TemplateBinding IsSubmenuOpen}"
AllowsTransparency="True"
Focusable="False"
PopupAnimation="Fade">
<Border
Name="SubmenuBorder"
SnapsToDevicePixels="True"
Style="{StaticResource StyleBlocsPopUpWithBorder}"
TextBlock.Foreground="{StaticResource CouleurTexteBouton}">
<ScrollViewer Style="{StaticResource FavsScrollViewer}" VerticalScrollBarVisibility="Auto" Margin="0,5" MaxHeight="400">
<StackPanel
IsItemsHost="True"
KeyboardNavigation.DirectionalNavigation="Cycle" />
</ScrollViewer>
</Border>
</Popup>
</Grid>
</Border>
</ControlTemplate>
The scrollviewer is working fine but the horizontal offset between each level is not working anymore if the parent has a scrollbar (my level 0 has only 7 items so doesn't have any scrollbar so the horizontal offset of the subitem works fine. But as you can see in the screen (left bloc is level 1 = subitems from my level 0, right bloc is level 2 of subitem and the horizontal offset isn't working due to the scrollbar.
How can I fix this ?
Here is the complete template/style of my contextmenu :
<Style x:Key="MenuItemStyle" TargetType="MenuItem">
<Setter Property="OverridesDefaultStyle" Value="True"/>
<Setter Property="Background" Value="Transparent" />
<Style.Triggers>
<Trigger Property="Role" Value="TopLevelHeader">
<Setter Property="Template"
Value="{StaticResource {x:Static MenuItem.TopLevelHeaderTemplateKey}}"/>
<Setter Property="Grid.IsSharedSizeScope" Value="true"/>
</Trigger>
<Trigger Property="Role" Value="TopLevelItem">
<Setter Property="Template"
Value="{StaticResource {x:Static MenuItem.TopLevelItemTemplateKey}}"/>
</Trigger>
<Trigger Property="Role" Value="SubmenuHeader">
<Setter Property="Template"
Value="{StaticResource {x:Static MenuItem.SubmenuHeaderTemplateKey}}"/>
</Trigger>
<Trigger Property="Role" Value="SubmenuItem">
<Setter Property="Template"
Value="{StaticResource {x:Static MenuItem.SubmenuItemTemplateKey}}"/>
</Trigger>
</Style.Triggers>
</Style>
<Style x:Key="StyleContext" TargetType="ContextMenu">
<Setter Property="SnapsToDevicePixels" Value="True"/>
<Setter Property="OverridesDefaultStyle" Value="True"/>
<Setter Property="Grid.IsSharedSizeScope" Value="true"/>
<Setter Property="HasDropShadow" Value="True"/>
<Setter Property="VerticalOffset" Value="10" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ContextMenu">
<Border
Name="Border"
Width="Auto" >
<StackPanel IsItemsHost="True"
KeyboardNavigation.DirectionalNavigation="Cycle"/>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="{x:Static MenuItem.SeparatorStyleKey}"
TargetType="Separator">
<Setter Property="Height" Value="1"/>
<Setter Property="Margin" Value="0,4,0,4"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Separator">
<Border BorderBrush="Green"
BorderThickness="1"/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<!-- TopLevelHeader -->
<ControlTemplate x:Key="{x:Static MenuItem.TopLevelHeaderTemplateKey}"
TargetType="MenuItem">
<Border Name="Border" Cursor="Hand">
<Grid>
<ContentPresenter
Margin="6,3,6,3"
Name="HeaderHost"
ContentSource="Header"
RecognizesAccessKey="True"
/>
<Popup
Name="Popup"
Placement="Bottom"
IsOpen="{TemplateBinding IsSubmenuOpen}"
AllowsTransparency="True"
Focusable="False"
PopupAnimation="Fade">
<Border
Name="SubmenuBorder"
SnapsToDevicePixels="True"
Background="Blue"
BorderBrush="Yellow"
BorderThickness="2" >
<StackPanel
IsItemsHost="True"
KeyboardNavigation.DirectionalNavigation="Cycle" />
</Border>
</Popup>
</Grid>
</Border>
</ControlTemplate>
<!-- TopLevelItem -->
<ControlTemplate
x:Key="{x:Static MenuItem.TopLevelItemTemplateKey}"
TargetType="MenuItem">
<Border Name="Border" Cursor="Hand" >
<Grid>
<ContentPresenter
Margin="6,3,6,3"
ContentSource="Header"
Name="content_text"
RecognizesAccessKey="True" />
</Grid>
</Border>
</ControlTemplate>
<!-- SubmenuItem
-->
<ControlTemplate
x:Key="{x:Static MenuItem.SubmenuItemTemplateKey}"
TargetType="MenuItem"
>
<Border
Name="Border"
BorderBrush="Transparent"
Background="Transparent"
BorderThickness="0" Cursor="Hand">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" SharedSizeGroup="Icon"/>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="Auto" SharedSizeGroup="Shortcut"/>
<ColumnDefinition Width="13"/>
</Grid.ColumnDefinitions>
<ContentPresenter
Name="Icon"
Margin="6,0,6,0"
VerticalAlignment="Center"
ContentSource="Icon"/>
<Border
Name="Check"
Width="13" Height="13"
Visibility="Collapsed"
Margin="6,0,6,0"
Background="Transparent"
BorderThickness="0"
BorderBrush="Transparent">
<Path
Name="CheckMark"
Width="7" Height="7"
Visibility="Hidden"
SnapsToDevicePixels="False"
StrokeThickness="2"
Data="M 0 0 L 7 7 M 0 7 L 7 0" />
</Border>
<ContentPresenter
Name="HeaderHost"
Grid.Column="1"
ContentSource="Header"
RecognizesAccessKey="True"/>
<TextBlock x:Name="InputGestureText"
Grid.Column="2"
Text="{TemplateBinding InputGestureText}"
Margin="5,2,0,2"
DockPanel.Dock="Right" />
</Grid>
</Border>
</ControlTemplate>
Thank you

WPF Scrollbar Style issue

I have a standard GridView and a standard TreeView, with no style / control templates implemented, just some basic customisation (background, foreground etc). I do, however, have a Scrollbar style that's set to apply to all scroll bars {x:Type ScrollBar}. It works perfectly, but I get a small white box between the scrollbars on both my TreeView and Image on other pages.
They're all hosted in a grid with a border or two - identically as far as I can tell. I've tried setting the border, foreground and background colors for just about every control on the page to no avail. I don't really want to define a TreeView style if possible, especially if that's not going to fix the issue.
Any ideas what I need to do to format that little white box? Code for my Scrollbar is below - it's fairly standard. I'll post the XAML for the windows themselves if necessary.
GridView
TreeView
<Style x:Key="ScrollBarLineButton"
TargetType="{x:Type RepeatButton}">
<Setter Property="SnapsToDevicePixels"
Value="True" />
<Setter Property="OverridesDefaultStyle"
Value="true" />
<Setter Property="Focusable"
Value="false" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type RepeatButton}">
<Border x:Name="Border"
Margin="1"
CornerRadius="2"
BorderThickness="1"
BorderBrush="{StaticResource ControlBorderBrush}"
Background="{StaticResource BackgroundBrush}">
<Path x:Name="Arrow"
HorizontalAlignment="Center"
VerticalAlignment="Center"
Data="{Binding Content,
RelativeSource={RelativeSource TemplatedParent}}"
Fill="White">
</Path>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="ScrollBarPageButton"
TargetType="{x:Type RepeatButton}">
<Setter Property="SnapsToDevicePixels"
Value="True" />
<Setter Property="OverridesDefaultStyle"
Value="true" />
<Setter Property="IsTabStop"
Value="false" />
<Setter Property="Focusable"
Value="false" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type RepeatButton}">
<Border Background="Transparent" />
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="ScrollBarThumb"
TargetType="{x:Type Thumb}">
<Setter Property="SnapsToDevicePixels"
Value="True" />
<Setter Property="OverridesDefaultStyle"
Value="true" />
<Setter Property="IsTabStop"
Value="false" />
<Setter Property="Focusable"
Value="false" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Thumb}">
<Border CornerRadius="4"
Background="{StaticResource ProxyRadial}"
BorderThickness="1"
BorderBrush="{StaticResource BorderBrush}"/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<ControlTemplate x:Key="VerticalScrollBar"
TargetType="{x:Type ScrollBar}">
<Grid>
<Grid.RowDefinitions>
<RowDefinition MaxHeight="18" />
<RowDefinition Height="0.00001*" />
<RowDefinition MaxHeight="18" />
</Grid.RowDefinitions>
<Border Grid.RowSpan="3"
CornerRadius="2"
Background="{StaticResource DarkBackgroundBrush}" />
<RepeatButton Grid.Row="0"
Style="{StaticResource ScrollBarLineButton}"
Height="18"
Command="ScrollBar.LineUpCommand"
Content="M 0 4 L 8 4 L 4 0 Z" />
<Track x:Name="PART_Track"
Grid.Row="1"
IsDirectionReversed="true">
<Track.DecreaseRepeatButton>
<RepeatButton Style="{StaticResource ScrollBarPageButton}"
Command="ScrollBar.PageUpCommand" />
</Track.DecreaseRepeatButton>
<Track.Thumb>
<Thumb Style="{StaticResource ScrollBarThumb}"
Margin="2,0"
Name="Thumb"/>
</Track.Thumb>
<Track.IncreaseRepeatButton>
<RepeatButton Style="{StaticResource ScrollBarPageButton}"
Command="ScrollBar.PageDownCommand" />
</Track.IncreaseRepeatButton>
</Track>
<RepeatButton Grid.Row="3"
Style="{StaticResource ScrollBarLineButton}"
Height="18"
Command="ScrollBar.LineDownCommand"
Content="M 0 0 L 4 4 L 8 0 Z" />
</Grid>
</ControlTemplate>
<ControlTemplate x:Key="HorizontalScrollBar"
TargetType="{x:Type ScrollBar}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition MaxWidth="18" />
<ColumnDefinition Width="0.00001*" />
<ColumnDefinition MaxWidth="18" />
</Grid.ColumnDefinitions>
<Border Grid.ColumnSpan="3"
CornerRadius="2"
Background="{StaticResource DarkBackgroundBrush}" />
<RepeatButton Grid.Column="0"
Style="{StaticResource ScrollBarLineButton}"
Width="18"
Command="ScrollBar.LineLeftCommand"
Content="M 4 0 L 4 8 L 0 4 Z" />
<Track x:Name="PART_Track"
Grid.Column="1"
IsDirectionReversed="False">
<Track.DecreaseRepeatButton>
<RepeatButton Style="{StaticResource ScrollBarPageButton}"
Command="ScrollBar.PageLeftCommand" />
</Track.DecreaseRepeatButton>
<Track.Thumb>
<Thumb Style="{StaticResource ScrollBarThumb}"
Margin="0,2"/>
</Track.Thumb>
<Track.IncreaseRepeatButton>
<RepeatButton Style="{StaticResource ScrollBarPageButton}"
Command="ScrollBar.PageRightCommand" />
</Track.IncreaseRepeatButton>
</Track>
<RepeatButton Grid.Column="3"
Style="{StaticResource ScrollBarLineButton}"
Width="18"
Command="ScrollBar.LineRightCommand"
Content="M 0 0 L 4 4 L 0 8 Z" />
</Grid>
</ControlTemplate>
<Style x:Key="{x:Type ScrollBar}"
TargetType="{x:Type ScrollBar}">
<Setter Property="SnapsToDevicePixels"
Value="True" />
<Setter Property="OverridesDefaultStyle"
Value="true" />
<Style.Triggers>
<Trigger Property="Orientation"
Value="Horizontal">
<Setter Property="Width"
Value="Auto" />
<Setter Property="Height"
Value="16" />
<Setter Property="Template"
Value="{StaticResource HorizontalScrollBar}" />
</Trigger>
<Trigger Property="Orientation"
Value="Vertical">
<Setter Property="Width"
Value="16" />
<Setter Property="Height"
Value="Auto" />
<Setter Property="Template"
Value="{StaticResource VerticalScrollBar}" />
</Trigger>
</Style.Triggers>
</Style>

If you try to copy ScrollViewerTemplate then you'll see this inside
<ControlTemplate x:Key="ScrollViewerControlTemplate1" TargetType="{x:Type ScrollViewer}">
<Grid x:Name="Grid" Background="{TemplateBinding Background}">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="*"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<Rectangle x:Name="Corner" Grid.Column="1" Fill="{DynamicResource {x:Static SystemColors.ControlBrushKey}}" Grid.Row="1"/>
<ScrollContentPresenter x:Name="PART_ScrollContentPresenter" CanContentScroll="{TemplateBinding CanContentScroll}" CanHorizontallyScroll="False" CanVerticallyScroll="False" ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" Grid.Column="0" Margin="{TemplateBinding Padding}" Grid.Row="0"/>
<ScrollBar x:Name="PART_VerticalScrollBar" AutomationProperties.AutomationId="VerticalScrollBar" Cursor="Arrow" Grid.Column="1" Maximum="{TemplateBinding ScrollableHeight}" Minimum="0" Grid.Row="0" Visibility="{TemplateBinding ComputedVerticalScrollBarVisibility}" Value="{Binding VerticalOffset, Mode=OneWay, RelativeSource={RelativeSource TemplatedParent}}" ViewportSize="{TemplateBinding ViewportHeight}"/>
<ScrollBar x:Name="PART_HorizontalScrollBar" AutomationProperties.AutomationId="HorizontalScrollBar" Cursor="Arrow" Grid.Column="0" Maximum="{TemplateBinding ScrollableWidth}" Minimum="0" Orientation="Horizontal" Grid.Row="1" Visibility="{TemplateBinding ComputedHorizontalScrollBarVisibility}" Value="{Binding HorizontalOffset, Mode=OneWay, RelativeSource={RelativeSource TemplatedParent}}" ViewportSize="{TemplateBinding ViewportWidth}"/>
</Grid>
</ControlTemplate>
And attention to this one
<Rectangle x:Name="Corner" Grid.Column="1" Fill="{DynamicResource {x:Static SystemColors.ControlBrushKey}}" Grid.Row="1"/>
Just change its Fill color

How to change the background of the MenuItem on MouseOver in WPF

Reading lots of stuff and asking a bunch of questions I have finally put together a new style for my Menu/MenuItem in WPF.
But I'm stuck where I want to change the background and border of a single submenu when it is highlighted.
In the image below, I want the blue background to be changed to say yellow!
Here is the complete working code so far:
<Window x:Class="WpfApplication12.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<LinearGradientBrush x:Key="DarkBrush" StartPoint="0.5,0" EndPoint="0.5,1">
<GradientBrush.GradientStops>
<GradientStopCollection>
<GradientStop Color="#939393" Offset="0.0"/>
<GradientStop Color="#717171" Offset="0.05"/>
<GradientStop Color="#606060" Offset="1.0"/>
</GradientStopCollection>
</GradientBrush.GradientStops>
</LinearGradientBrush>
<LinearGradientBrush x:Key="Clicked" StartPoint="0.5,0" EndPoint="0.5,1">
<GradientBrush.GradientStops>
<GradientStopCollection>
<GradientStop Color="#363636" Offset="0.0"/>
<GradientStop Color="#393939" Offset="1.0"/>
</GradientStopCollection>
</GradientBrush.GradientStops>
</LinearGradientBrush>
<Style x:Key="{x:Type Menu}" TargetType="Menu">
<Setter Property="OverridesDefaultStyle" Value="True"/>
<Setter Property="SnapsToDevicePixels" Value="True"/>
<Setter Property="Foreground" Value="White"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Menu">
<Border x:Name="MainMenu" Background="#535353">
<StackPanel ClipToBounds="True" Orientation="Horizontal" IsItemsHost="True"/>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<ControlTemplate x:Key="MenuItemControlTemplate1" TargetType="{x:Type MenuItem}">
<Border x:Name="templateRoot"
BorderBrush="#535353"
CornerRadius="3"
BorderThickness="1"
Background="{TemplateBinding Background}"
SnapsToDevicePixels="True">
<Grid VerticalAlignment="Center">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<ContentPresenter ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}" Grid.Column="1" ContentStringFormat="{TemplateBinding HeaderStringFormat}" ContentSource="Header" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
<Popup x:Name="PART_Popup" AllowsTransparency="True" Focusable="False" IsOpen="{Binding IsSubmenuOpen, RelativeSource={RelativeSource TemplatedParent}}" PopupAnimation="{DynamicResource {x:Static SystemParameters.MenuPopupAnimationKey}}" Placement="Bottom">
<Border x:Name="SubMenuBorder" BorderBrush="#595959" BorderThickness="1" Background="#3A3A3A" Padding="2">
<ScrollViewer x:Name="SubMenuScrollViewer" Style="{DynamicResource {ComponentResourceKey ResourceId=MenuScrollViewer, TypeInTargetAssembly={x:Type FrameworkElement}}}">
<Grid RenderOptions.ClearTypeHint="Enabled">
<Canvas HorizontalAlignment="Left" Height="0" VerticalAlignment="Top" Width="0">
<Rectangle x:Name="OpaqueRect" Fill="{Binding Background, ElementName=SubMenuBorder}" Height="{Binding ActualHeight, ElementName=SubMenuBorder}" Width="{Binding ActualWidth, ElementName=SubMenuBorder}"/>
</Canvas>
<ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Cycle" Grid.IsSharedSizeScope="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" KeyboardNavigation.TabNavigation="Cycle"/>
</Grid>
</ScrollViewer>
</Border>
</Popup>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsSuspendingPopupAnimation" Value="True">
<Setter Property="PopupAnimation" TargetName="PART_Popup" Value="None"/>
</Trigger>
<Trigger Property="IsHighlighted" Value="True">
<Setter Property="Background" TargetName="templateRoot" Value="{StaticResource DarkBrush}"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="#2C2C2C"/>
<Setter Property="BorderThickness" TargetName="templateRoot" Value="1"></Setter>
</Trigger>
<Trigger Property="CanContentScroll" SourceName="SubMenuScrollViewer" Value="False">
<Setter Property="Canvas.Top" TargetName="OpaqueRect" Value="{Binding VerticalOffset, ElementName=SubMenuScrollViewer}"/>
<Setter Property="Canvas.Left" TargetName="OpaqueRect" Value="{Binding HorizontalOffset, ElementName=SubMenuScrollViewer}"/>
</Trigger>
<Trigger Property="IsKeyboardFocusWithin" Value="True">
<Setter TargetName="templateRoot" Property="Background" Value="{StaticResource Clicked}" />
<Setter Property="Header" Value="Test" />
<Setter Property="BorderBrush" Value="#2C2C2C"></Setter>
<Setter Property="BorderThickness" Value="1"></Setter>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Window.Resources>
<Grid Background="#535353">
<Menu Width="100" Height="22" Margin="10, 10, 5, 5" HorizontalAlignment="Left" Background="White" VerticalAlignment="Top">
<MenuItem Header="_File" Name="mm" Template="{DynamicResource MenuItemControlTemplate1}">
<MenuItem Header="_New"/>
<MenuItem Header="_Open"/>
<MenuItem Header="_Close"/>
</MenuItem>
<MenuItem Header="_Edit" Template="{DynamicResource MenuItemControlTemplate1}">
<MenuItem Header="_Copy"/>
<MenuItem Header="_Cut"/>
<MenuItem Header="_Paste"/>
</MenuItem>
</Menu>
</Grid>
</Window>

Just make another template. Here is one with the background yellow, border orange and text black.
Look at MenuItemControlTemplate2, the IsHighlighted trigger.
<Window x:Class="WpfApplication12.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<LinearGradientBrush x:Key="DarkBrush" StartPoint="0.5,0" EndPoint="0.5,1">
<GradientBrush.GradientStops>
<GradientStopCollection>
<GradientStop Color="#939393" Offset="0.0"/>
<GradientStop Color="#717171" Offset="0.05"/>
<GradientStop Color="#606060" Offset="1.0"/>
</GradientStopCollection>
</GradientBrush.GradientStops>
</LinearGradientBrush>
<LinearGradientBrush x:Key="Clicked" StartPoint="0.5,0" EndPoint="0.5,1">
<GradientBrush.GradientStops>
<GradientStopCollection>
<GradientStop Color="#363636" Offset="0.0"/>
<GradientStop Color="#393939" Offset="1.0"/>
</GradientStopCollection>
</GradientBrush.GradientStops>
</LinearGradientBrush>
<Style x:Key="{x:Type Menu}" TargetType="Menu">
<Setter Property="OverridesDefaultStyle" Value="True"/>
<Setter Property="SnapsToDevicePixels" Value="True"/>
<Setter Property="Foreground" Value="White"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Menu">
<Border x:Name="MainMenu" Background="#535353">
<StackPanel ClipToBounds="True" Orientation="Horizontal" IsItemsHost="True"/>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<ControlTemplate x:Key="MenuItemControlTemplate1" TargetType="{x:Type MenuItem}">
<Border x:Name="templateRoot"
BorderBrush="#535353"
CornerRadius="3"
BorderThickness="1"
Background="{TemplateBinding Background}"
SnapsToDevicePixels="True">
<Grid VerticalAlignment="Center">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<ContentPresenter ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}" Grid.Column="1" ContentStringFormat="{TemplateBinding HeaderStringFormat}" ContentSource="Header" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
<Popup x:Name="PART_Popup" AllowsTransparency="True" Focusable="False" IsOpen="{Binding IsSubmenuOpen, RelativeSource={RelativeSource TemplatedParent}}" PopupAnimation="{DynamicResource {x:Static SystemParameters.MenuPopupAnimationKey}}" Placement="Bottom">
<Border x:Name="SubMenuBorder" BorderBrush="#595959" BorderThickness="1" Background="#3A3A3A" Padding="2">
<ScrollViewer x:Name="SubMenuScrollViewer" Style="{DynamicResource {ComponentResourceKey ResourceId=MenuScrollViewer, TypeInTargetAssembly={x:Type FrameworkElement}}}">
<Grid RenderOptions.ClearTypeHint="Enabled">
<Canvas HorizontalAlignment="Left" Height="0" VerticalAlignment="Top" Width="0">
<Rectangle x:Name="OpaqueRect" Fill="{Binding Background, ElementName=SubMenuBorder}" Height="{Binding ActualHeight, ElementName=SubMenuBorder}" Width="{Binding ActualWidth, ElementName=SubMenuBorder}"/>
</Canvas>
<ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Cycle" Grid.IsSharedSizeScope="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" KeyboardNavigation.TabNavigation="Cycle"/>
</Grid>
</ScrollViewer>
</Border>
</Popup>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsSuspendingPopupAnimation" Value="True">
<Setter Property="PopupAnimation" TargetName="PART_Popup" Value="None"/>
</Trigger>
<Trigger Property="IsHighlighted" Value="True">
<Setter Property="Background" TargetName="templateRoot" Value="{StaticResource DarkBrush}"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="#2C2C2C"/>
<Setter Property="BorderThickness" TargetName="templateRoot" Value="1"></Setter>
</Trigger>
<Trigger Property="CanContentScroll" SourceName="SubMenuScrollViewer" Value="False">
<Setter Property="Canvas.Top" TargetName="OpaqueRect" Value="{Binding VerticalOffset, ElementName=SubMenuScrollViewer}"/>
<Setter Property="Canvas.Left" TargetName="OpaqueRect" Value="{Binding HorizontalOffset, ElementName=SubMenuScrollViewer}"/>
</Trigger>
<Trigger Property="IsKeyboardFocusWithin" Value="True">
<Setter TargetName="templateRoot" Property="Background" Value="{StaticResource Clicked}" />
<Setter Property="Header" Value="Test" />
<Setter Property="BorderBrush" Value="#2C2C2C"></Setter>
<Setter Property="BorderThickness" Value="1"></Setter>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
<ControlTemplate x:Key="MenuItemControlTemplate2" TargetType="{x:Type MenuItem}">
<Border x:Name="templateRoot" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" SnapsToDevicePixels="True">
<Grid Margin="-1">
<Grid.ColumnDefinitions>
<ColumnDefinition MinWidth="22" SharedSizeGroup="MenuItemIconColumnGroup" Width="Auto"/>
<ColumnDefinition Width="13"/>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="30"/>
<ColumnDefinition SharedSizeGroup="MenuItemIGTColumnGroup" Width="Auto"/>
<ColumnDefinition Width="20"/>
</Grid.ColumnDefinitions>
<ContentPresenter x:Name="Icon" Content="{TemplateBinding Icon}" ContentSource="Icon" HorizontalAlignment="Center" Height="16" Margin="3" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="Center" Width="16"/>
<Border x:Name="GlyphPanel" BorderBrush="#FF26A0DA" BorderThickness="1" Background="#3D26A0DA" ClipToBounds="False" HorizontalAlignment="Center" Height="22" Margin="-1,0,0,0" Visibility="Hidden" VerticalAlignment="Center" Width="22">
<Path x:Name="Glyph" Data="F1M10,1.2L4.7,9.1 4.5,9.1 0,5.2 1.3,3.5 4.3,6.1 8.3,0 10,1.2z" Fill="#FF212121" FlowDirection="LeftToRight" Height="11" Width="10"/>
</Border>
<ContentPresenter x:Name="menuHeaderContainer" ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}" Grid.Column="2" ContentStringFormat="{TemplateBinding HeaderStringFormat}" ContentSource="Header" HorizontalAlignment="Left" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="Center"/>
<TextBlock x:Name="menuGestureText" Grid.Column="4" Margin="{TemplateBinding Padding}" Opacity="0.7" Text="{TemplateBinding InputGestureText}" VerticalAlignment="Center"/>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="Icon" Value="{x:Null}">
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsChecked" Value="True">
<Setter Property="Visibility" TargetName="GlyphPanel" Value="Visible"/>
<Setter Property="Visibility" TargetName="Icon" Value="Collapsed"/>
</Trigger>
<Trigger Property="IsHighlighted" Value="True">
<Setter Property="BorderBrush" TargetName="templateRoot" Value="Orange"/>
<Setter Property="Background" TargetName="templateRoot" Value="Yellow"/>
<Setter Property="TextBlock.Foreground" TargetName="menuHeaderContainer" Value="Black" />
</Trigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="TextElement.Foreground" TargetName="templateRoot" Value="#FF707070"/>
<Setter Property="Fill" TargetName="Glyph" Value="#FF707070"/>
</Trigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsHighlighted" Value="True"/>
<Condition Property="IsEnabled" Value="False"/>
</MultiTrigger.Conditions>
<Setter Property="Background" TargetName="templateRoot" Value="#0A000000"/>
<Setter Property="BorderBrush" TargetName="templateRoot" Value="#21000000"/>
</MultiTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Window.Resources>
<Grid Background="#535353">
<Menu Width="100" Height="22" Margin="10, 10, 5, 5" HorizontalAlignment="Left" Background="White" VerticalAlignment="Top">
<MenuItem Header="_File" Name="mm" Template="{DynamicResource MenuItemControlTemplate1}">
<MenuItem Header="_New" Template="{DynamicResource MenuItemControlTemplate2}" />
<MenuItem Header="_Open" Template="{DynamicResource MenuItemControlTemplate2}" />
<MenuItem Header="_Close" Template="{DynamicResource MenuItemControlTemplate2}" />
</MenuItem>
<MenuItem Header="_Edit" Template="{DynamicResource MenuItemControlTemplate1}">
<MenuItem Header="_Copy" Template="{DynamicResource MenuItemControlTemplate2}" />
<MenuItem Header="_Cut" Template="{DynamicResource MenuItemControlTemplate2}" />
<MenuItem Header="_Paste" Template="{DynamicResource MenuItemControlTemplate2}" />
</MenuItem>
</Menu>
</Grid>