Yi-Lun Luo - MSFT
Hello, the problem maybe, RadioButton¡¯s Background only affects the small circle, not its Content. So the change may be difficult to notice. You can instead set a Background for the BulletDecorator in ControlTemplate:
<ControlTemplate TargetType="{x:Type RadioButton}">
<BulletDecorator x:Name="bulletDecorator" Background="#FFFF0000">
<BulletDecorator.Bullet>
<Microsoft_Windows_Themes:BulletChrome Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" IsChecked="{TemplateBinding IsChecked}" IsRound="true" RenderMouseOver="{TemplateBinding IsMouseOver}" RenderPressed="{TemplateBinding IsPressed}"/>
</BulletDecorator.Bullet>
<ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" RecognizesAccessKey="True"/>
</BulletDecorator>
<ControlTemplate.Triggers>
<Trigger Property="IsChecked" Value="True">
<Setter Property="Background" TargetName="bulletDecorator" Value="#FF0000FF"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>