WPF 丸いタブ

こうかな

<Style TargetType="TabItem">
    <Setter Property="Foreground" Value="#cfd5dc" />
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="TabItem">
                <Border Name="Border" BorderThickness="1,1,1,0" BorderBrush="Gainsboro" CornerRadius="4,4,0,0" Margin="2,0">
                    <ContentPresenter x:Name="ContentSite"
                                    VerticalAlignment="Center"
                                    HorizontalAlignment="Center"
                                    ContentSource="Header"
                                    Margin="10,2"/>
                </Border>
                <ControlTemplate.Triggers>
                    <Trigger Property="IsSelected" Value="True">
                        <Setter TargetName="Border" Property="Background" Value="Gray" />
                    </Trigger>
                    <Trigger Property="IsSelected" Value="False">
                        <Setter TargetName="Border" Property="Background" Value="Black" />
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>