Tabitem WPF change tabitem border instead of background color

Question

I was reviewing the code:

<Window x:Class="WpfTutorialSamples.Misc_controls.StyledTabItemsSample"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="StyledTabItemsSample" Height="150" Width="250">
    <Grid>
        <TabControl Margin="10" BorderThickness="0" Background="LightGray">
            <TabControl.Resources>
                <Style TargetType="TabItem">
                    <Setter Property="Template">
                        <Setter.Value>
                            <ControlTemplate TargetType="TabItem">
                                <Grid Name="Panel">
                                    <ContentPresenter x:Name="ContentSite"
                                        VerticalAlignment="Center"
                                        HorizontalAlignment="Center"
                                        ContentSource="Header"
                                        Margin="10,2"/>
                                </Grid>
                                <ControlTemplate.Triggers>
                                    <Trigger Property="IsSelected" Value="True">
                                        <Setter TargetName="Panel" Property="Background" Value="LightSkyBlue" />
                                    </Trigger>
                                    <Trigger Property="IsSelected" Value="False">
                                        <Setter TargetName="Panel" Property="Background" Value="White" />
                                    </Trigger>
                                </ControlTemplate.Triggers>
                            </ControlTemplate>
                        </Setter.Value>
                    </Setter>
                </Style>
            </TabControl.Resources>
            <TabItem Header="General">
                <Label Content="Content goes here..." />
            </TabItem>
            <TabItem Header="Security" />
            <TabItem Header="Details" />
        </TabControl>
    </Grid>
</Window>

The result is:

My goal is to not change the background of the tabItem but instead change the bottom border of the select tab. For example is General is selected, it would have the text "General" underlined and colored blue.

Answer 1

You can accomplish this by replacing Grid which hosts ContentPresenter with Border and then make the Trigger to change its BorderBrush like the following.

<TabControl Margin="10" BorderThickness="0" Background="LightGray">
    <TabControl.Resources>
        <Style TargetType="TabItem">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="TabItem">
                        <Border Name="Panel"
                                BorderThickness="0,0,0,2"
                                BorderBrush="Transparent">
                            <ContentPresenter x:Name="ContentSite"
                                VerticalAlignment="Center"
                                HorizontalAlignment="Center"
                                ContentSource="Header"
                                Margin="10,2"/>
                        </Border>

                        <ControlTemplate.Triggers>
                            <Trigger Property="IsSelected" Value="True">
                                <Setter TargetName="Panel" Property="BorderBrush" Value="LightSkyBlue" />
                            </Trigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </TabControl.Resources>

    <TabItem Header="General">
        <Label Content="Content goes here..." />
    </TabItem>
    <TabItem Header="Security" />
    <TabItem Header="Details" />
</TabControl>