Avalonia/samples/XamlTestApplicationPcl/TabItem.paml

<Styles xmlns="https://github.com/perspex">
  <Style Selector="TabItem">
    <Setter Property="FontSize" Value="16"/>
    <Setter Property="Foreground" Value="Gray"/>
    <Setter Property="Template">
      <ControlTemplate>
        <ContentPresenter Name="PART_HeaderPresenter" Content="{TemplateBinding Header}"/>
      </ControlTemplate>
    </Setter>
  </Style>
  <Style Selector="TabItem:selected">
    <Setter Property="Foreground" Value="Black"/>
  </Style>
</Styles>