Avalonia/samples/UnloadableAssemblyLoadContext/UnloadableAssemblyLoadContextPlug/ControlStyle.axaml

<Styles xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        >
  <Design.PreviewWith>
    <Border Padding="20">
      <!-- Add Controls for Previewer Here -->
    </Border>
  </Design.PreviewWith>

  <StyleInclude Source="TestControl.axaml"></StyleInclude>
</Styles>