Develop Reference

WPF Style for ToggleButton as Chevron - how to remove annoying thin lines

While trying to style a ToggleButton with a Chevron shape, i cannot get rid of the annoying thin lines which are rendered, which can be seen at the screenshot.
Can anyone help out here?
Style:
<Style x:Key="ChevronButtonStyle" TargetType="{x:Type ToggleButton}" >
<Setter Property="Margin" Value="2" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ToggleButton}">
<StackPanel Orientation="Horizontal" Margin="0,0,-10,0" Height="30" ClipToBounds="True">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<Path Grid.Column="0" Name="ChevronLeft" Data="M0,0 10,0 10,30 0,30 10,15" Fill="{TemplateBinding Background}"/>
<Grid Grid.Column="1" >
<Rectangle Name="ChevronBack" Fill="{TemplateBinding Background}" Margin="0,0,0,0"/>
<TextBlock Grid.Column="2" Name="ChevronText" Text="{TemplateBinding Content}" Margin="10,5" VerticalAlignment="Center" />
</Grid>
<Path Grid.Column="2" Name="ChevronRight" Data="M0,0 10,15 0,30" Fill="{TemplateBinding Background}" />
</Grid>
</StackPanel>
<ControlTemplate.Triggers>
<Trigger Property="IsChecked" Value="True">
<Setter TargetName="ChevronLeft" Property="Fill" Value="{StaticResource Petrol}" />
<Setter TargetName="ChevronBack" Property="Fill" Value="{StaticResource Petrol}" />
<Setter TargetName="ChevronRight" Property="Fill" Value="{StaticResource Petrol}" />
<Setter TargetName="ChevronText" Property="Foreground" Value="White" />
</Trigger>
<Trigger Property="IsEnabled" Value="False">
<Setter TargetName="ChevronLeft" Property="Fill" Value="DarkGray" />
<Setter TargetName="ChevronBack" Property="Fill" Value="DarkGray" />
<Setter TargetName="ChevronRight" Property="Fill" Value="DarkGray" />
<Setter TargetName="ChevronText" Property="Foreground" Value="Black" />
</Trigger>
<Trigger Property="IsChecked" Value="False">
<Setter TargetName="ChevronText" Property="Foreground" Value="Black" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

Theses artifacts should disappear when you set UseLayoutRounding or SnapsToDevicePixels on the parent Grid:
<Grid UseLayoutRounding="True">
Or
<Grid SnapsToDevicePixels="True">

c# WPF remove ListBox Content ScrollBar Gap - Photo included

I have a custom scroll bar template that is set like this:
<ListBox.Resources>
<Style TargetType="{x:Type ScrollBar}" BasedOn="{StaticResource ScrollBar_New}"/>
</ListBox.Resources>
I am trying to move those buttons closer to the scroll bar and not sure if there is a setting I am missing someplace. Do I need to also modify the listbox control style? I am still new to WPF and not sure if there is something I am missing here. I have already tried <Setter Property="Width" Value="10"/> to see if that that alters it, but it does not.
Style Used:
<Style x:Key="ScrollThumbs" TargetType="{x:Type Thumb}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Thumb}">
<Grid x:Name="Grid">
<Rectangle HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Width="Auto" Height="Auto" Fill="Transparent" />
<Border x:Name="Rectangle1" CornerRadius="5" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Width="Auto" Height="Auto"
Background="{TemplateBinding Background}" />
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="Tag" Value="Horizontal">
<Setter TargetName="Rectangle1" Property="Width" Value="Auto" />
<Setter TargetName="Rectangle1" Property="Height" Value="7" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<!--ScrollBars-->
<Style x:Key="ScrollBar_Rounded" TargetType="{x:Type ScrollBar}">
<Setter Property="Stylus.IsFlicksEnabled" Value="false" />
<Setter Property="Foreground" Value="#FFFCC515" />
<Setter Property="Background" Value="Transparent" />
<Setter Property="Width" Value="8" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ScrollBar}">
<Grid x:Name="GridRoot" Width="8" Background="{TemplateBinding Background}">
<Grid.RowDefinitions>
<RowDefinition Height="0.00001*" />
</Grid.RowDefinitions>
<Track x:Name="PART_Track" Grid.Row="0" IsDirectionReversed="true" Focusable="false">
<Track.Thumb>
<Thumb x:Name="Thumb" Background="{TemplateBinding Foreground}" Style="{DynamicResource ScrollThumbs}" />
</Track.Thumb>
<Track.IncreaseRepeatButton>
<RepeatButton x:Name="PageUp" Command="ScrollBar.PageDownCommand" Opacity="0" Focusable="false" />
</Track.IncreaseRepeatButton>
<Track.DecreaseRepeatButton>
<RepeatButton x:Name="PageDown" Command="ScrollBar.PageUpCommand" Opacity="0" Focusable="false" />
</Track.DecreaseRepeatButton>
</Track>
</Grid>
<ControlTemplate.Triggers>
<Trigger SourceName="Thumb" Property="IsMouseOver" Value="true">
<Setter Value="{DynamicResource ButtonSelectBrush}" TargetName="Thumb" Property="Background" />
</Trigger>
<Trigger SourceName="Thumb" Property="IsDragging" Value="true">
<Setter Value="{DynamicResource DarkBrush}" TargetName="Thumb" Property="Background" />
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter TargetName="Thumb" Property="Visibility" Value="Collapsed" />
</Trigger>
<Trigger Property="Orientation" Value="Horizontal">
<Setter TargetName="GridRoot" Property="LayoutTransform">
<Setter.Value>
<RotateTransform Angle="-90" />
</Setter.Value>
</Setter>
<Setter TargetName="PART_Track" Property="LayoutTransform">
<Setter.Value>
<RotateTransform Angle="-90" />
</Setter.Value>
</Setter>
<Setter Property="Width" Value="Auto" />
<Setter Property="Height" Value="8" />
<Setter TargetName="Thumb" Property="Tag" Value="Horizontal" />
<Setter TargetName="PageDown" Property="Command" Value="ScrollBar.PageLeftCommand" />
<Setter TargetName="PageUp" Property="Command" Value="ScrollBar.PageRightCommand" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

Ok after a little bit of testing i've come to my conclusion. In your style there are several things that affect the width of the scrollbar. To avoid to mess up the everything hardconding width
<Style x:Key="ScrollThumbs" TargetType="{x:Type Thumb}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Thumb}">
<Grid x:Name="Grid">
<Rectangle HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Width="Auto" Height="Auto" Fill="Transparent" />
<Border Margin="3" x:Name="Rectangle1" CornerRadius="5" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Width="Auto" Height="Auto"
Background="{TemplateBinding Background}" />
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="Tag" Value="Horizontal">
<Setter TargetName="Rectangle1" Property="Width" Value="Auto" />
<Setter TargetName="Rectangle1" Property="Height" Value="Auto" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<!--ScrollBars-->
<Style x:Key="ScrollBar_Rounded" TargetType="{x:Type ScrollBar}">
<Setter Property="Stylus.IsFlicksEnabled" Value="false" />
<Setter Property="Foreground" Value="#FFFCC515" />
<Setter Property="Background" Value="Transparent" />
<Setter Property="Width" Value="8" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ScrollBar}">
<Grid x:Name="GridRoot" Width="Auto" Background="{TemplateBinding Background}">
<Grid.RowDefinitions>
<RowDefinition Height="0.00001*" />
</Grid.RowDefinitions>
<Track x:Name="PART_Track" Grid.Row="0" IsDirectionReversed="true" Focusable="false">
<Track.Thumb>
<Thumb x:Name="Thumb" Background="{TemplateBinding Foreground}" Style="{DynamicResource ScrollThumbs}" />
</Track.Thumb>
<Track.IncreaseRepeatButton>
<RepeatButton x:Name="PageUp" Command="ScrollBar.PageDownCommand" Opacity="0" Focusable="false" />
</Track.IncreaseRepeatButton>
<Track.DecreaseRepeatButton>
<RepeatButton x:Name="PageDown" Command="ScrollBar.PageUpCommand" Opacity="0" Focusable="false" />
</Track.DecreaseRepeatButton>
</Track>
</Grid>
<ControlTemplate.Triggers>
<Trigger SourceName="Thumb" Property="IsMouseOver" Value="true">
<Setter Value="{DynamicResource ButtonSelectBrush}" TargetName="Thumb" Property="Background" />
</Trigger>
<Trigger SourceName="Thumb" Property="IsDragging" Value="true">
<Setter Value="{DynamicResource DarkBrush}" TargetName="Thumb" Property="Background" />
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter TargetName="Thumb" Property="Visibility" Value="Collapsed" />
</Trigger>
<Trigger Property="Orientation" Value="Horizontal">
<Setter TargetName="GridRoot" Property="LayoutTransform">
<Setter.Value>
<RotateTransform Angle="-90" />
</Setter.Value>
</Setter>
<Setter TargetName="PART_Track" Property="LayoutTransform">
<Setter.Value>
<RotateTransform Angle="-90" />
</Setter.Value>
</Setter>
<Setter Property="Width" Value="Auto" />
<Setter Property="Height" Value="8" />
<Setter TargetName="Thumb" Property="Tag" Value="Horizontal" />
<Setter TargetName="PageDown" Property="Command" Value="ScrollBar.PageLeftCommand" />
<Setter TargetName="PageUp" Property="Command" Value="ScrollBar.PageRightCommand" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Shortly What i did was to :
Change the setter of the Height Property for Rectangle 1 to auto (this cause to have horizonatal and vertical scrollbar to look the same)
Set GridRoot Width to auto In the ScrollBar_Rounded style (this cause the scrollbar to get all the available space)
Use a margin in the Rectangle1 to the desired space (this will help you to decide how much space should be displayed between your scrollbars and the content of the listbox)

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.

c# wpf Changing color of thumb/scrollbox RichTextBox

i want to change color of thumb/scrollbox from RichTextBox.
I'm using, but it doesn't changes color of thumb
<RichTextBox x:Name="richTextBox" HorizontalAlignment="Left" Height="285" VerticalAlignment="Top" Width="880" VerticalScrollBarVisibility="Visible" IsReadOnly="True" Foreground="#FFA02626" Background="{x:Null}">
<RichTextBox.Resources>
<Style TargetType="ScrollBar">
<Setter Property="Foreground" Value="Red"/>
</Style>
</RichTextBox.Resources>
</RichTextBox>

I don't find any easy way to do this. But you can define your custom ScrollBar style.
Here is my example I just wrote. It's look so ugly because I'm going to bed.
<RichTextBox>
<RichTextBox.Resources>
<Style x:Key="ScrollThumb" TargetType="{x:Type Thumb}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Thumb}">
<Rectangle x:Name="Rectangle1" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Fill="Transparent" SnapsToDevicePixels="True"/>
<ControlTemplate.Triggers>
<Trigger Property="Tag" Value="Horizontal">
<Setter TargetName="Rectangle1" Property="Width" Value="Auto" />
<Setter TargetName="Rectangle1" Property="Height" Value="7" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="{x:Type ScrollBar}" TargetType="{x:Type ScrollBar}">
<Setter Property="Stylus.IsFlicksEnabled" Value="false" />
<Setter Property="Foreground" Value="Red" />
<Setter Property="Background" Value="DarkGray" />
<Setter Property="Width" Value="10" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ScrollBar}">
<Grid x:Name="GridRoot" Width="19" Background="{TemplateBinding Background}">
<Grid.RowDefinitions>
<RowDefinition Height="0.00001*" />
</Grid.RowDefinitions>
<Track x:Name="PART_Track" Grid.Row="0" IsDirectionReversed="true" Focusable="false">
<Track.Thumb>
<Thumb x:Name="Thumb" Background="{TemplateBinding Foreground}" Style="{DynamicResource ScrollThumbs}" />
</Track.Thumb>
<Track.IncreaseRepeatButton>
<RepeatButton x:Name="PageUp" Command="ScrollBar.PageDownCommand" Opacity="0" Focusable="false" />
</Track.IncreaseRepeatButton>
<Track.DecreaseRepeatButton>
<RepeatButton x:Name="PageDown" Command="ScrollBar.PageUpCommand" Opacity="0" Focusable="false" />
</Track.DecreaseRepeatButton>
</Track>
</Grid>
<ControlTemplate.Triggers>
<Trigger SourceName="Thumb" Property="IsMouseOver" Value="true">
<Setter Value="{DynamicResource ButtonSelectBrush}" TargetName="Thumb" Property="Background" />
</Trigger>
<Trigger SourceName="Thumb" Property="IsDragging" Value="true">
<Setter Value="{DynamicResource DarkBrush}" TargetName="Thumb" Property="Background" />
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter TargetName="Thumb" Property="Visibility" Value="Collapsed" />
</Trigger>
<Trigger Property="Orientation" Value="Horizontal">
<Setter TargetName="GridRoot" Property="LayoutTransform">
<Setter.Value>
<RotateTransform Angle="-90" />
</Setter.Value>
</Setter>
<Setter TargetName="PART_Track" Property="LayoutTransform">
<Setter.Value>
<RotateTransform Angle="-90" />
</Setter.Value>
</Setter>
<Setter Property="Width" Value="Auto" />
<Setter Property="Height" Value="12" />
<Setter TargetName="Thumb" Property="Tag" Value="Horizontal" />
<Setter TargetName="PageDown" Property="Command" Value="ScrollBar.PageLeftCommand" />
<Setter TargetName="PageUp" Property="Command" Value="ScrollBar.PageRightCommand" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</RichTextBox.Resources>
</RichTextBox>
You can improve it follow MS guide here : https://msdn.microsoft.com/en-us/library/ms742173(v=vs.110).aspx
Or here: https://www.google.com

WPF ComboBox Dropdropdown SelectedItem Background

I use a FlatComboBox Style. Everything is working just like I want it to. I just to change one thing, the Background of the Selected Item in the DropDown. How can I change the color? I'm using Windows 8 and .Net 4. But the solution should also run with older Windows Versions. I save this Style in a ResourceDictionary.
My Code for the Style:
<!-- Flat ComboBox -->
<SolidColorBrush x:Key="ComboBoxNormalBorderBrush" Color="#e3e9ef" />
<SolidColorBrush x:Key="ComboBoxNormalBackgroundBrush" Color="#fff" />
<SolidColorBrush x:Key="ComboBoxDisabledForegroundBrush" Color="#888" />
<SolidColorBrush x:Key="ComboBoxDisabledBackgroundBrush" Color="#eee" />
<SolidColorBrush x:Key="ComboBoxDisabledBorderBrush" Color="#888" />
<ControlTemplate TargetType="ToggleButton" x:Key="ComboBoxToggleButtonTemplate">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition />
<ColumnDefinition Width="20" />
</Grid.ColumnDefinitions>
<Border Grid.ColumnSpan="2" Name="Border" BorderBrush="{StaticResource ComboBoxNormalBorderBrush}" CornerRadius="0" BorderThickness="1, 1, 1, 1" Background="{StaticResource ComboBoxNormalBackgroundBrush}" />
<Border Grid.Column="1" Margin="1, 1, 1, 1" BorderBrush="#444" Name="ButtonBorder" CornerRadius="0, 0, 0, 0" BorderThickness="0, 0, 0, 0" Background="{StaticResource ComboBoxNormalBackgroundBrush}" />
<Path Name="Arrow" Grid.Column="1" Data="M0,0 L0,2 L4,6 L8,2 L8,0 L4,4 z" HorizontalAlignment="Center" Fill="#444" VerticalAlignment="Center" />
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="UIElement.IsMouseOver" Value="True">
<Setter Property="Panel.Background" TargetName="ButtonBorder" Value="WhiteSmoke"/>
</Trigger>
<Trigger Property="ToggleButton.IsChecked" Value="True">
<Setter Property="Panel.Background" TargetName="ButtonBorder" Value="WhiteSmoke"/>
<Setter Property="Shape.Fill" TargetName="Arrow" Value="#FF8D979E"/>
</Trigger>
<Trigger Property="UIElement.IsEnabled" Value="False">
<Setter Property="Panel.Background" TargetName="Border" Value="{StaticResource ComboBoxDisabledBackgroundBrush}"/>
<Setter Property="Panel.Background" TargetName="ButtonBorder" Value="{StaticResource ComboBoxDisabledBackgroundBrush}"/>
<Setter Property="Border.BorderBrush" TargetName="ButtonBorder" Value="{StaticResource ComboBoxDisabledBorderBrush}"/>
<Setter Property="TextElement.Foreground" Value="{StaticResource ComboBoxDisabledForegroundBrush}"/>
<Setter Property="Shape.Fill" TargetName="Arrow" Value="#999"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
<Style x:Key="ComboBoxFlatStyle" TargetType="{x:Type ComboBox}">
<Setter Property="UIElement.SnapsToDevicePixels" Value="True"/>
<Setter Property="FrameworkElement.OverridesDefaultStyle" Value="True"/>
<Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto"/>
<Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto"/>
<Setter Property="ScrollViewer.CanContentScroll" Value="True"/>
<Setter Property="TextElement.Foreground" Value="Black"/>
<Setter Property="FrameworkElement.FocusVisualStyle" Value="{x:Null}"/>
<Setter Property="Control.Template">
<Setter.Value>
<ControlTemplate TargetType="ComboBox">
<Grid>
<ToggleButton Name="ToggleButton" Grid.Column="2" ClickMode="Press" Focusable="False" IsChecked="{Binding Path=IsDropDownOpen, RelativeSource={RelativeSource TemplatedParent}, Mode=TwoWay}"
Template="{StaticResource ComboBoxToggleButtonTemplate}"/>
<ContentPresenter Name="ContentSite" Margin="5, 3, 23, 3" IsHitTestVisible="False" HorizontalAlignment="Left" VerticalAlignment="Center" Content="{TemplateBinding ComboBox.SelectionBoxItem}"
ContentTemplate="{TemplateBinding ComboBox.SelectionBoxItemTemplate}" ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}"/>
<TextBox Name="PART_EditableTextBox" Margin="3, 3, 23, 3" IsReadOnly="{TemplateBinding IsReadOnly}" Visibility="Hidden" Background="Transparent" HorizontalAlignment="Left" VerticalAlignment="Center"
Focusable="True" >
<TextBox.Template>
<ControlTemplate TargetType="TextBox" >
<Border Name="PART_ContentHost" Focusable="False" />
</ControlTemplate>
</TextBox.Template>
</TextBox>
<!-- Popup showing items -->
<Popup Name="Popup" Placement="Bottom" Focusable="False" AllowsTransparency="True" IsOpen="{TemplateBinding ComboBox.IsDropDownOpen}" PopupAnimation="Slide">
<Grid Name="DropDown" SnapsToDevicePixels="True" MinWidth="{TemplateBinding FrameworkElement.ActualWidth}" MaxHeight="{TemplateBinding ComboBox.MaxDropDownHeight}">
<Border Name="DropDownBorder" Background="White" Margin="0, 1, 0, 0" CornerRadius="0" BorderThickness="1,1,1,1" BorderBrush="{StaticResource ComboBoxNormalBorderBrush}"/>
<ScrollViewer Margin="4" SnapsToDevicePixels="True">
<ItemsPresenter KeyboardNavigation.DirectionalNavigation="Contained" />
</ScrollViewer>
</Grid>
</Popup>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="ItemsControl.HasItems" Value="False">
<Setter Property="FrameworkElement.MinHeight" TargetName="DropDownBorder" Value="95"/>
</Trigger>
<Trigger Property="UIElement.IsEnabled" Value="False">
<Setter Property="TextElement.Foreground" Value="{StaticResource ComboBoxDisabledForegroundBrush}"/>
</Trigger>
<Trigger Property="ItemsControl.IsGrouping" Value="True">
<Setter Property="ScrollViewer.CanContentScroll" Value="False"/>
</Trigger>
<Trigger Property="ComboBox.IsEditable" Value="True">
<Setter Property="KeyboardNavigation.IsTabStop" Value="False"/>
<Setter Property="UIElement.Visibility" TargetName="PART_EditableTextBox" Value="Visible"/>
<Setter Property="UIElement.Visibility" TargetName="ContentSite" Value="Hidden"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<!-- End of Flat ComboBox -->
Working Solution
I had to replace the TextBlock with a ContentPresenter to get it working, when the ItemsSource is a DataTable.
<Setter Property="ItemContainerStyle">
<Setter.Value>
<Style TargetType="ComboBoxItem">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ComboBoxItem">
<Border Background="{TemplateBinding Background}" BorderThickness="0" Margin="2">
<!--<TextBlock Text="{TemplateBinding Content}" Margin="2" />-->
<ContentPresenter />
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
<Trigger Property="IsSelected" Value="True">
<Setter Property="Background" Value="Red" />
</Trigger>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Background" Value="AliceBlue" />
</Trigger>
</Style.Triggers>
</Style>
</Setter.Value>
</Setter>

You can define a style (with a trigger) for the ItemContainerStyle of your ComboBox.
<Style x:Key="ComboBoxFlatStyle" TargetType="{x:Type ComboBox}">
<Setter Property="UIElement.SnapsToDevicePixels" Value="True"/>
<!-- .... -->
<Setter Property="ItemContainerStyle">
<Setter.Value>
<Style TargetType="ComboBoxItem">
<Style.Triggers>
<Trigger Property="IsSelected" Value="True">
<Setter Property="Background" Value="Gray" />
</Trigger>
</Style.Triggers>
</Style>
</Setter.Value>
</Setter>
</Style>
<!-- End of Flat ComboBox -->
EDIT
I post my Whole Window XAML:
<Window x:Class="WpfApplication1.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:sys="clr-namespace:System;assembly=mscorlib"
Title="Window1" Height="300" Width="300">
<Window.Resources>
<!-- Flat ComboBox -->
<SolidColorBrush x:Key="ComboBoxNormalBorderBrush" Color="#e3e9ef" />
<SolidColorBrush x:Key="ComboBoxNormalBackgroundBrush" Color="#fff" />
<SolidColorBrush x:Key="ComboBoxDisabledForegroundBrush" Color="#888" />
<SolidColorBrush x:Key="ComboBoxDisabledBackgroundBrush" Color="#eee" />
<SolidColorBrush x:Key="ComboBoxDisabledBorderBrush" Color="#888" />
<ControlTemplate TargetType="ToggleButton" x:Key="ComboBoxToggleButtonTemplate">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition />
<ColumnDefinition Width="20" />
</Grid.ColumnDefinitions>
<Border Grid.ColumnSpan="2" Name="Border" BorderBrush="{StaticResource ComboBoxNormalBorderBrush}" CornerRadius="0" BorderThickness="1, 1, 1, 1" Background="{StaticResource ComboBoxNormalBackgroundBrush}" />
<Border Grid.Column="1" Margin="1, 1, 1, 1" BorderBrush="#444" Name="ButtonBorder" CornerRadius="0, 0, 0, 0" BorderThickness="0, 0, 0, 0" Background="{StaticResource ComboBoxNormalBackgroundBrush}" />
<Path Name="Arrow" Grid.Column="1" Data="M0,0 L0,2 L4,6 L8,2 L8,0 L4,4 z" HorizontalAlignment="Center" Fill="#444" VerticalAlignment="Center" />
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="UIElement.IsMouseOver" Value="True">
<Setter Property="Panel.Background" TargetName="ButtonBorder" Value="WhiteSmoke"/>
</Trigger>
<Trigger Property="ToggleButton.IsChecked" Value="True">
<Setter Property="Panel.Background" TargetName="ButtonBorder" Value="WhiteSmoke"/>
<Setter Property="Shape.Fill" TargetName="Arrow" Value="#FF8D979E"/>
</Trigger>
<Trigger Property="UIElement.IsEnabled" Value="False">
<Setter Property="Panel.Background" TargetName="Border" Value="{StaticResource ComboBoxDisabledBackgroundBrush}"/>
<Setter Property="Panel.Background" TargetName="ButtonBorder" Value="{StaticResource ComboBoxDisabledBackgroundBrush}"/>
<Setter Property="Border.BorderBrush" TargetName="ButtonBorder" Value="{StaticResource ComboBoxDisabledBorderBrush}"/>
<Setter Property="TextElement.Foreground" Value="{StaticResource ComboBoxDisabledForegroundBrush}"/>
<Setter Property="Shape.Fill" TargetName="Arrow" Value="#999"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
<Style x:Key="ComboBoxFlatStyle" TargetType="{x:Type ComboBox}">
<Setter Property="UIElement.SnapsToDevicePixels" Value="True"/>
<Setter Property="FrameworkElement.OverridesDefaultStyle" Value="True"/>
<Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto"/>
<Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto"/>
<Setter Property="ScrollViewer.CanContentScroll" Value="True"/>
<Setter Property="TextElement.Foreground" Value="Black"/>
<Setter Property="FrameworkElement.FocusVisualStyle" Value="{x:Null}"/>
<Setter Property="Control.Template">
<Setter.Value>
<ControlTemplate TargetType="ComboBox">
<Grid>
<ToggleButton Name="ToggleButton" Grid.Column="2" ClickMode="Press" Focusable="False" IsChecked="{Binding Path=IsDropDownOpen, RelativeSource={RelativeSource TemplatedParent}, Mode=TwoWay}"
Template="{StaticResource ComboBoxToggleButtonTemplate}"/>
<ContentPresenter Name="ContentSite" Margin="5, 3, 23, 3" IsHitTestVisible="False" HorizontalAlignment="Left" VerticalAlignment="Center" Content="{TemplateBinding ComboBox.SelectionBoxItem}"
ContentTemplate="{TemplateBinding ComboBox.SelectionBoxItemTemplate}" ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}"/>
<TextBox Name="PART_EditableTextBox" Margin="3, 3, 23, 3" IsReadOnly="{TemplateBinding IsReadOnly}" Visibility="Hidden" Background="Transparent" HorizontalAlignment="Left" VerticalAlignment="Center"
Focusable="True" >
<TextBox.Template>
<ControlTemplate TargetType="TextBox" >
<Border Name="PART_ContentHost" Focusable="False" />
</ControlTemplate>
</TextBox.Template>
</TextBox>
<!-- Popup showing items -->
<Popup Name="Popup" Placement="Bottom" Focusable="False" AllowsTransparency="True" IsOpen="{TemplateBinding ComboBox.IsDropDownOpen}" PopupAnimation="Slide">
<Grid Name="DropDown" SnapsToDevicePixels="True" MinWidth="{TemplateBinding FrameworkElement.ActualWidth}" MaxHeight="{TemplateBinding ComboBox.MaxDropDownHeight}">
<Border Name="DropDownBorder" Background="White" Margin="0, 1, 0, 0" CornerRadius="0" BorderThickness="1,1,1,1" BorderBrush="{StaticResource ComboBoxNormalBorderBrush}"/>
<ScrollViewer Margin="4" SnapsToDevicePixels="True">
<ItemsPresenter KeyboardNavigation.DirectionalNavigation="Contained" />
</ScrollViewer>
</Grid>
</Popup>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="ItemsControl.HasItems" Value="False">
<Setter Property="FrameworkElement.MinHeight" TargetName="DropDownBorder" Value="95"/>
</Trigger>
<Trigger Property="UIElement.IsEnabled" Value="False">
<Setter Property="TextElement.Foreground" Value="{StaticResource ComboBoxDisabledForegroundBrush}"/>
</Trigger>
<Trigger Property="ItemsControl.IsGrouping" Value="True">
<Setter Property="ScrollViewer.CanContentScroll" Value="False"/>
</Trigger>
<Trigger Property="ComboBox.IsEditable" Value="True">
<Setter Property="KeyboardNavigation.IsTabStop" Value="False"/>
<Setter Property="UIElement.Visibility" TargetName="PART_EditableTextBox" Value="Visible"/>
<Setter Property="UIElement.Visibility" TargetName="ContentSite" Value="Hidden"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
<Setter Property="ItemContainerStyle">
<Setter.Value>
<Style TargetType="ComboBoxItem">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ComboBoxItem">
<Border Background="{TemplateBinding Background}" BorderThickness="0" Margin="2">
<TextBlock Text="{TemplateBinding Content}" Margin="2" />
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
<Trigger Property="IsSelected" Value="True">
<Setter Property="Background" Value="Red" />
</Trigger>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Background" Value="AliceBlue" />
</Trigger>
</Style.Triggers>
</Style>
</Setter.Value>
</Setter>
</Style>
<!-- End of Flat ComboBox -->
</Window.Resources>
<DockPanel>
<ComboBox Height="24" Width="190" Style="{StaticResource ComboBoxFlatStyle}">
<ComboBox.Items>
<sys:String>One</sys:String>
<sys:String>Two</sys:String>
<sys:String>Three</sys:String>
<sys:String>Four</sys:String>
<sys:String>Five</sys:String>
</ComboBox.Items>
</ComboBox>
</DockPanel>
</Window>
And this is the result:

Add the following to your ComboBox-Style:
<Style.Resources>
<SolidColorBrush x:Key="{x:Static SystemColors.HighlightBrushKey}" Color="Green" />
</Style.Resources>
In an application where I have a ComboBox with changed background-brush of the selected item I use the following style-definition:
<Style TargetType="{x:Type ComboBox}">
<Setter Property="IsReadOnly" Value="True" />
<Setter Property="IsEditable" Value="False" />
<Setter Property="ItemTemplate">
<Setter.Value>
<DataTemplate>
<TextBlock Text="{Binding}" Foreground="Black" />
</DataTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
<Trigger Property="IsSelected" Value="True">
<Setter Property="BorderBrush" Value="{StaticResource fokusBrush}" />
<Setter Property="BorderThickness" Value="1.3" />
<Setter Property="Margin" Value="2.5" />
</Trigger>
</Style.Triggers>
<Style.Resources>
<SolidColorBrush x:Key="{x:Static SystemColors.HighlightBrushKey}" Color="#C2E58F" />
</Style.Resources>
</Style>
Maybe that works for you.
I've created a new WPF-Project with your style in the App.xaml. When I add the following to <Style x:Key="ComboBoxFlatStyle" TargetType="{x:Type ComboBox}"> the background of the selected item in the drop-down is red:
<Style.Resources>
<SolidColorBrush x:Key="{x:Static SystemColors.HighlightBrushKey}" Color="Red"></SolidColorBrush>
</Style.Resources>
My dummy-ComboBox looks like:
<ComboBox HorizontalAlignment="Left" Margin="101,64,0,0" VerticalAlignment="Top" Width="120"
Style="{StaticResource ComboBoxFlatStyle}" >
<ComboBox.Items>
<ComboBoxItem Content="Blub1"></ComboBoxItem>
<ComboBoxItem Content="Blub1"></ComboBoxItem>
<ComboBoxItem Content="Blub1"></ComboBoxItem>
</ComboBox.Items>
</ComboBox>
Now if you open the drop down and move the cursor over an item -> the background of this item is red.