Avalonia/samples/ControlCatalog/Pages/NavigationDemoPage.xaml

<ContentPage Theme="{StaticResource ScrollPage}" xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             Header="Navigation Page"
             x:Class="ControlCatalog.Pages.NavigationDemoPage">
  <Grid>
    <NavigationPage x:Name="SampleNav">
      <NavigationPage.Styles>
        <Style Selector="NavigationPage#SampleNav /template/ Border#PART_NavigationBar">
          <Setter Property="Background" Value="Transparent" />
        </Style>
      </NavigationPage.Styles>
    </NavigationPage>
</Grid>
</ContentPage>