The RadToggleButton can be styled by creating an appropriate Style (or use Implicit Style) and setting it to the Style property of the control.
You have two options:
Example 1: Create custom implicit style
<Window.Resources> <Style TargetType="telerik:RadToggleButton"> <Setter Property="Opacity" Value="0.5"/> <Setter Property="BorderBrush" Value="Red"/> <Setter Property="BorderThickness" Value="2"/> </Style> </Window.Resources> <Grid> <telerik:RadToggleButton Width="200" Height="200" Content="Check" VerticalAlignment="Center" HorizontalAlignment="Center"/> </Grid>
If you prefer, you can manually extract the needed style from the respective XAML file in the Themes.Implicit folder of your Telerik UI installation and modify its code to suit your needs.
If you choose to define the style in the App.xaml file, it would be available for the entire application. This allows you to define a style only once and then reuse it where needed.
Note that when changing a ControlTemplate you should include all required parts. Even if your code compiles, some of the functionality may be impacted due to the omission of the required parts. The required parts are usually marked with the prefix "PART_".