Avalonia/samples/ControlCatalog/TransparentStyles.axaml

<Styles xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:ControlCatalog"
        x:Class="ControlCatalog.TransparentStyles">

  <!-- Only used when WindowTransparencyLevel != None -->

  <Styles.Resources>
    <SolidColorBrush x:Key="NavigationBarBackground" Color="Transparent" />
    <SolidColorBrush x:Key="TitleBarBackgroundBrush" Color="Transparent" />
    <SolidColorBrush x:Key="ExpanderHeaderBackground" Color="Transparent" />
    <SolidColorBrush x:Key="ExpanderHeaderBackgroundPointerOver" Color="#33808080" />
    <SolidColorBrush x:Key="ExpanderHeaderBackgroundPressed" Color="#66808080" />
  </Styles.Resources>

  <Style Selector="local|MainView">
    <Setter Property="Background" Value="Transparent" />
    <Setter Property="DrawerBackground" Value="Transparent" />

    <Style Selector="^ TabbedPage">
      <Setter Property="Background" Value="Transparent" />
    </Style>
  </Style>

</Styles>