Avalonia/src/Avalonia.Themes.Default/TabStripItem.xaml

<Styles xmlns="https://github.com/avaloniaui">
  <Style Selector="TabStripItem">
    <Setter Property="Background" Value="Transparent"/>
    <Setter Property="FontSize" Value="{StyleResource FontSizeLarge}"/>
    <Setter Property="Foreground" Value="{StyleResource ThemeForegroundLightBrush}"/>
    <Setter Property="Template">
      <ControlTemplate>
        <ContentPresenter Name="PART_ContentPresenter"
                          Background="{TemplateBinding Background}"
                          BorderBrush="{TemplateBinding BorderBrush}"
                          BorderThickness="{TemplateBinding BorderThickness}"
                          Content="{TemplateBinding Content}"
                          ContentTemplate="{TemplateBinding ContentTemplate}"
                          Padding="{TemplateBinding Padding}"/>
      </ControlTemplate>
    </Setter>
  </Style>
  <Style Selector="TabStripItem:selected">
    <Setter Property="Foreground" Value="{StyleResource ThemeForegroundBrush}"/>
  </Style>
</Styles>