<Window.Resources>
        <Style TargetType="TabItem">
            <Style.Triggers>
                <Trigger Property="IsSelected" Value="True">
                    <Trigger.Setters>
                        <Setter Property="Background" Value="Black"/>
                    </Trigger.Setters>
                </Trigger>
            </Style.Triggers>
        </Style>
    </Window.Resources>