Один из вариантов - скопировать стиль элемента табуляции по умолчанию, затем изменить его радиус угла границы, фон и т. Д., Затем дать ему ключ, а затем применить свой стиль к фактическому элементу управления, например:
<StackPanel>
<Grid>
<Grid.Resources>
<Style x:Key="CustomTabItem" TargetType="TabItem">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TabItem}">
<Grid x:Name="Root"
Background="AntiqueWhite">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="SelectionStates">
<VisualState x:Name="Unselected" />
<VisualState x:Name="Selected">
<Storyboard>
<ColorAnimationUsingKeyFrames Storyboard.TargetName="Border"
Storyboard.TargetProperty="(Panel.Background).
(GradientBrush.GradientStops)[1].(GradientStop.Color)">
<EasingColorKeyFrame KeyTime="0"
Value="{DynamicResource ControlPressedColor}" />
</ColorAnimationUsingKeyFrames>
<ThicknessAnimationUsingKeyFrames Storyboard.TargetProperty="(Border.BorderThickness)"
Storyboard.TargetName="Border">
<EasingThicknessKeyFrame KeyTime="0"
Value="1,1,1,0" />
</ThicknessAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal" />
<VisualState x:Name="MouseOver" />
<VisualState x:Name="Disabled">
<Storyboard>
<ColorAnimationUsingKeyFrames Storyboard.TargetName="Border"
Storyboard.TargetProperty="(Panel.Background).
(GradientBrush.GradientStops)[1].(GradientStop.Color)">
<EasingColorKeyFrame KeyTime="0"
Value="{DynamicResource DisabledControlDarkColor}" />
</ColorAnimationUsingKeyFrames>
<ColorAnimationUsingKeyFrames Storyboard.TargetName="Border"
Storyboard.TargetProperty="(Border.BorderBrush).
(SolidColorBrush.Color)">
<EasingColorKeyFrame KeyTime="0"
Value="{DynamicResource DisabledBorderLightColor}"/>
</ColorAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Border x:Name="Border"
Margin="0,0,-4,0"
BorderThickness="1,1,1,1"
CornerRadius="10">
<Border.BorderBrush>
<SolidColorBrush Color="Black" />
</Border.BorderBrush>
<Border.Background>
<LinearGradientBrush StartPoint="0,0"
EndPoint="0,1">
<LinearGradientBrush.GradientStops>
<GradientStopCollection>
<GradientStop Color="{DynamicResource ControlLightColor}"
Offset="0.0" />
<GradientStop Color="{DynamicResource ControlMediumColor}"
Offset="1.0" />
</GradientStopCollection>
</LinearGradientBrush.GradientStops>
</LinearGradientBrush>
</Border.Background>
<ContentPresenter x:Name="ContentSite"
VerticalAlignment="Center"
HorizontalAlignment="Center"
ContentSource="Header"
Margin="12,2,12,2"
RecognizesAccessKey="True"/>
</Border>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsSelected"
Value="True">
<Setter Property="Panel.ZIndex"
Value="100" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Grid.Resources>
<TabControl Name="TabControl1" TabStripPlacement="Left" Margin="-6,46,-14,-453">
<TabItem Style="{StaticResource CustomTabItem}">
<TabItem.Header>
<TextBlock Text="A" />
</TabItem.Header>
<Canvas>
</Canvas>
</TabItem>
<TabItem Style="{StaticResource CustomTabItem}">
<TabItem.Header>
<TextBlock Text="B" />
</TabItem.Header>
<Canvas>
</Canvas>
</TabItem>
<TabItem Style="{StaticResource CustomTabItem}">
<TabItem.Header>
<TextBlock Text="C" />
</TabItem.Header>
<Canvas>
</Canvas>
</TabItem>
</TabControl>
</Grid>
</StackPanel>