Avalonia/samples/ControlCatalog/Pages/DrawerDemoPage.xaml

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="ControlCatalog.Pages.DrawerDemoPage">
  <NavigationPage x:Name="SampleNav">
    <NavigationPage.Styles>
      <Style Selector="NavigationPage#SampleNav /template/ Border#PART_NavigationBar">
        <Setter Property="Background" Value="Transparent" />
      </Style>
    </NavigationPage.Styles>
  </NavigationPage>
</UserControl>