<ControlTheme x:Key="{x:Type WindowDrawnDecorations}" TargetType="WindowDrawnDecorations">
    <Setter Property="Template">
        <WindowDrawnDecorationsTemplate>
            <WindowDrawnDecorationsContent>
                <WindowDrawnDecorationsContent.Overlay>
                    <!-- Overlay: only interactive caption buttons -->
                    <ContentControl>
                        <StackPanel VerticalAlignment="Top"
                                    HorizontalAlignment="Right"
                                    Height="{TemplateBinding TitleBarHeight}"
                                    Orientation="Horizontal"
                                    Background="Black">
                            <Rectangle x:Name="A" Fill="Red" Width="30" />
                            <Rectangle x:Name="B" Fill="Green" Width="30" />
                            <Rectangle x:Name="C" Fill="Blue" Width="30" />
                        </StackPanel>
                    </ContentControl>
                </WindowDrawnDecorationsContent.Overlay>
    
            </WindowDrawnDecorationsContent>
        </WindowDrawnDecorationsTemplate>
    </Setter>
</ControlTheme>

<Style Selector="WindowDrawnDecorations /template/ Rectangle#B">
    <Setter Property="IsVisible" Value="False" />
</Style>

<Window ...
        ExtendClientAreaToDecorationsHint="True"
        ExtendClientAreaTitleBarHeightHint="30">
    ...
</Window>