Editopia/Editopia.Editor/MainView.xaml

<UserControl xmlns="https://github.com/avaloniaui"
        xmlns:pages="clr-namespace:Editopia.Editor.Pages"
        xmlns:sidebar="clr-namespace:Editopia.Editor.Sidebar"
        xmlns:editopia.control="clr-namespace:Editopia.Control"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

  <TabControl Classes="sidebar" Name="Sidebar">
    <TabItem Header="File">
      <pages:FilePage/>
    </TabItem>
    <TabItem Header="Trainer">
      <pages:EditTrainer/>
    </TabItem>
  </TabControl>
</UserControl>