Shaker/Avalonia/SukiUI.Dock/ToolTabStrip.axaml

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <Design.PreviewWith>
    <Border Padding="20">
      <ToolTabStrip>
        <ToolTabStripItem>Item 1</ToolTabStripItem>
        <ToolTabStripItem>Item 2</ToolTabStripItem>
        <ToolTabStripItem IsEnabled="False">Disabled</ToolTabStripItem>
      </ToolTabStrip>
    </Border>
  </Design.PreviewWith>

  <ControlTheme x:Key="{x:Type ToolTabStrip}" TargetType="ToolTabStrip">

    <Setter Property="Background" Value="Transparent" />
    <Setter Property="Focusable" Value="False" />
    <Setter Property="ClipToBounds" Value="False" />
    <Setter Property="ZIndex" Value="1" />

    <Setter Property="Template">
      <ControlTemplate>
       
        <DockPanel>
          <Border Name="PART_Border"
                  Background="{TemplateBinding Background}"
                  BorderBrush="Red"
                  BorderThickness="0"
                  CornerRadius="{TemplateBinding CornerRadius}"
                  Padding="{TemplateBinding Padding}"
                  DockPanel.Dock="Left">
            <ItemsPresenter Name="PART_ItemsPresenter"
                            ItemsPanel="{TemplateBinding ItemsPanel}"/>
          </Border>
          <Border Name="PART_BorderFill" />
        </DockPanel>
          
      </ControlTemplate>
    </Setter>

    <Setter Property="ItemsPanel">
      <ItemsPanelTemplate>
        <StackPanel Orientation="Horizontal" ClipToBounds="False" />
      </ItemsPanelTemplate>
    </Setter>

    <Style Selector="^:singleitem /template/ ItemsPresenter#PART_ItemsPresenter">
      <Setter Property="IsVisible" Value="False" />
    </Style>

    <Style Selector="^/template/ Border#PART_BorderFill">
      <Setter Property="BorderBrush" Value="{DynamicResource DockThemeBorderLowBrush}" />
      <Setter Property="BorderThickness" Value="0 0 0  0" />
    </Style>

  </ControlTheme>

</ResourceDictionary>