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I have a RepeatButton in a Toolbar. I want it to have the same visual appearance as the ToolBar-Buttons. But ToolBar.ButtonStyleKey doesn't work because of the different TargetType. So where are the built-in styles defined (for various Themes) and how can I clone the ToolBar.ButtonStyle for RepeatButton?

They are defined in the C:WindowsMicrosoft.NETFramework64v4.0.30319WPFPresentationFramework* assemblies. You could use a decompiler such as dotPeek to extract the templates, or you can right-click on a control in design mode in Visual Studio or in Blend and choose Edit Template->Edit a Copy to copy the default template into your XAML markup.

I tried this and I got the Template for ToolBar, but not the style for ToolBarButton. The search ends at ToolBarButtonStyleKey, but can't find ToolBarButtonStyle for WPF.

It's right there:

<Style x:Key="x:Static ToolBar.ButtonStyleKey" TargetType="x:Type Button">
 <Setter Property="Control.Foreground" Value="DynamicResource x:Static SystemColors.ControlTextBrushKey"/>
 <Setter Property="Control.Padding" Value="2"/>
 <Setter Property="Control.BorderThickness" Value="1"/>
 <Setter Property="Control.Background" Value="Transparent"/>
 <Setter Property="Control.BorderBrush" Value="Transparent"/>
 <Setter Property="FrameworkElement.HorizontalAlignment" Value="Center"/>
 <Setter Property="FrameworkElement.VerticalAlignment" Value="Center"/>
 <Setter Property="Control.HorizontalContentAlignment" Value="Center"/>
 <Setter Property="Control.VerticalContentAlignment" Value="Center"/>
 <Setter Property="Control.Template">
 <Setter.Value>
 <ControlTemplate TargetType="x:Type Button">
 <Border Name="Bd" Background="TemplateBinding Control.Background"
 BorderBrush="TemplateBinding Control.BorderBrush"
 BorderThickness="TemplateBinding Control.BorderThickness"
 Padding="TemplateBinding Control.Padding" SnapsToDevicePixels="true">
 <ContentPresenter HorizontalAlignment="TemplateBinding Control.HorizontalContentAlignment"
 VerticalAlignment="TemplateBinding Control.VerticalContentAlignment"
 SnapsToDevicePixels="TemplateBinding UIElement.SnapsToDevicePixels"/>
 </Border>
 <ControlTemplate.Triggers>
 <Trigger Property="UIElement.IsMouseOver" Value="true">
 <Setter TargetName="Bd" Value="StaticResource ƻ" Property="Border.BorderBrush"/>
 <Setter TargetName="Bd" Value="StaticResource ƺ" Property="Border.Background"/>
 </Trigger>
 <Trigger Property="UIElement.IsKeyboardFocused" Value="true">
 <Setter TargetName="Bd" Value="StaticResource ƻ" Property="Border.BorderBrush"/>
 <Setter TargetName="Bd" Value="StaticResource ƺ" Property="Border.Background"/>
 </Trigger>
 <Trigger Property="ButtonBase.IsPressed" Value="true">
 <Setter TargetName="Bd" Value="StaticResource ƾ" Property="Border.BorderBrush"/>
 <Setter TargetName="Bd" Value="StaticResource ƽ" Property="Border.Background"/>
 </Trigger>
 <Trigger Property="UIElement.IsEnabled" Value="false">
 <Setter Value="DynamicResource x:Static SystemColors.GrayTextBrushKey" Property="Control.Foreground"/>
 </Trigger>
 </ControlTemplate.Triggers>
 </ControlTemplate>
 </Setter.Value>
 </Setter>