Develop Reference

How to apply style in WPF to all controls without overriding their original styles?

I want to create a style, which will normalize margins for all controls in a specific scenario:
<Style TargetType="FrameworkElement" x:Key="MyStyle">
<Setter Property="Margin" Value="{StaticResource DialogItemsExceptTopMargin}" />
</Style>
I have then a couple of different controls: textboxes, comboboxes, checkboxes etc., to which I want to apply this style.
However, when I do that, their look is immediately reverted to the platform style (I am applying a style from 3rd party library). How can I define my style so that the original styles are kept intact?
Mind: I know, that I can use BasedOn for specific type:
<Style TargetType="TextBox" BasedOn="{StaticResource {x:Type TextBox}}">
...
</Style>
This however would require me to create completely separate styles for all possible controls, which I may use. I tried this trick with FrameworkElement and Control, but I got an error, that there is no such StaticResource.

WPF Override Style Value

I'm using MaterialDesignInXaml for WPF which provides 3rd party controls and styles. I need to edit one of these styles by changing one property.
I am using an Expander control which has a template creating a bunch of child controls. I've discovered the child 'Border' control (4 layers deep) has the property (padding) which I need to set to zero.
See this output from Snoop showing the property I need to change:
Link to image
My question is how can I do this? I've tried extending the style used by the control as follows, but it isn't changing anything so I assume I'm doing something wrong?
<Style TargetType="{x:Type Expander}"
x:Key="MaterialDesignExpanderHeadless"
BasedOn="{StaticResource MaterialDesignExpander}">
<Style.Resources>
<Style TargetType="{x:Type Border}">
<Setter Property="Padding" Value="0"></Setter>
</Style>
</Style.Resources>
</Style>
I am able to use the style like this. And I know this is working for sure:
<Expander Header="Header Content" Style="{StaticResource MaterialDesignExpanderHeadless}">
Some Content
</Expander>

You're right, this method should work. Something else is setting the border's padding.
Snoop is telling you the padding is defined by the parent template, which could be the HeaderSite (ToggleButton).
You could try to extend the ToggleButton style (BasedOn) or redefine it locally.

Override specific properties of a style

I'm creating a new style for the TabControl including a new ItemContainerStyle for the items. The new style works fine, except that I need the possibility to add features to the ItemContainerStyle when using the style. In detail this is the Header property
<TabControl x:Name="myTabControl" SelectionChanged="myTabControl_SelectionChanged">
<TabControl.ItemContainerStyle>
<Style TargetType="TabItem">
<Setter Property="Header" Value="{Binding Title}"></Setter>
</Style>
</TabControl.ItemContainerStyle>
</TabControl>
which works OK. But it overrides the style completely. the BasedOn property would help, but I don't have access to the key of the ItemContainerStyle since it is embedded in the TabControls' style. How can I simply update a property of the style without overriding the style completely?
Thanks

There is a bit of a difference in the way styles in WPF compared to css. In Wpf they work completely off of inheritance, this is the basic document on how styles work. So if a key is not provided for you I think you are out of luck when using the BasedOn inheritance.
However, Microsoft does provide a useful utility in Visual Studio Blend. In the Objects and Timelines Window you right click then select an "edit a style", This will do all the heavy lifting for you. If you are going to be doing a lot of small changes on the style I would suggest you make a copy, and give it a Key, then use the BasedOn property to make your small changes that you want.
I Hope this helps.

You can base your style on the implicitly applied default style:
<Style TargetType="TabItem" BasedOn="{StaticResource {x:Type TabItem}}">

How can I override BorderBrush?

I have a style to a textbox, and I want in the application layer override de BorderBrush of this style.
I tried: d:LayoutOverrides="BorderBrush". But this doesn't work.
I want the same style, but with a red BorderBrush.
How can I do this please?
Thank you.

as you mentioned Blend in your tags: you can right-click a textbox search templates and create new ones from the existing (Edit a Copy). This will extract the complete definition of the textbox and you can change everything you want there.
Here is everything explained step-by-step: Create or edit a control template

Base your style on your old one and change the Border brush
<Style TargetType="TextBox" BasedOn="{StaticResource oldBrushKey}">
<Setter Property="BorderBrush" Value="Red" />
</Style>
or if it's the default style you want to override use
BasedOn="{StaticResource {x:Type TextBox}}"

How to add a trigger to a WPF custom control without overriding the existing style?

I am creating a simple custom control extending from toggle button that allows the user to specify checked and unchecked content directly in XAML. It works well but it is based on a trigger, and I don't know how to define the trigger except in a style. If I define the style, then I lose anything set outside of the custom control.
What I would like to be able to do is just append this trigger to any existing style set elsewhere on the control.
Here's the XAML for the style/trigger.
<ToggleButton.Style>
<Style TargetType="{x:Type ToggleButton}" BasedOn="{StaticResource {x:Type ToggleButton}}">
<Setter Property="Content" Value="{Binding RelativeSource={RelativeSource Self}, Path=UncheckedContent}" />
<Style.Triggers>
<Trigger Property="IsChecked" Value="True">
<Setter Property="Content"
Value="{Binding RelativeSource={RelativeSource Self}, Path=CheckedContent}" />
</Trigger>
</Style.Triggers>
</Style>
</ToggleButton.Style>
I tried inheriting the style via the BasedOn with a default type but it won't work if the custom control has an explicit style set by its parent. I also considered EventTriggers but I do not believe there would be an event to initialize the control.
Thanks for any help anyone can offer. :)

Just to clear things up on the terminology here: A user control is a control that derives from the UserControl class. If I understood you right you derived from ToggleButton to add the UncheckedContent and CheckedContent properties. In that case you have created a custom control. It's always easier to follow if we agree on common terminology :)
As far as I know you can not do such a generic style inheritance in XAML. You always have to specify explicitly what style a another style is based upon. Your style can either be based on the default style for ToggleButton or on a specific other style. If you can't build a style inheritance chain that respects that, this approach won't work.
But since you have a custom control, couldn't you write a default style for it that is based on the default toggle button style like this?
<Style TargetType="{x:Type CustomToggleButton}"
BasedOn="{StaticResource {x:Type ToggleButton}}">
Then whenever you apply an explicit style to a toggle button you would specify that it is based on the default toggle button style.
Also you could write a (default) control template for your new toggle button in Themes\Generic.xaml that contains the above triggers. In blend you can get a copy of the default template for toggle button ("Edit Template"->"Edit a Copy") so you can make sure that your toggle button looks exactly like the normal toggle button. Then incorporate the triggers above into that template.
BTW: you do not have to create a new control just to add new properties. You can add new properties to an existing control using attached properties. They can be used from XAML just like normal properties.