Use {x:Null} instead of transparent for popup root

src/Avalonia.Themes.Default/PopupRoot.xaml

@@ -1,7 +1,7 @@
 <Style xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Selector="PopupRoot">
-  <Setter Property="Background" Value="Transparent"/>
+  <Setter Property="Background" Value="{x:Null}"/>
   <Setter Property="TransparencyLevelHint" Value="Transparent" />
   <Setter Property="Foreground" Value="{DynamicResource ThemeForegroundBrush}"/>
   <Setter Property="FontSize" Value="{DynamicResource FontSizeNormal}"/>

src/Avalonia.Themes.Fluent/Controls/PopupRoot.xaml

@@ -1,7 +1,7 @@
 <Styles xmlns="https://github.com/avaloniaui"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
   <Style Selector="PopupRoot">
-    <Setter Property="Background" Value="Transparent"/>
+    <Setter Property="Background" Value="{x:Null}"/>
     <Setter Property="TransparencyLevelHint" Value="Transparent" />
     <Setter Property="Foreground" Value="{DynamicResource SystemControlForegroundBaseHighBrush}"/>
     <Setter Property="FontSize" Value="{DynamicResource ControlContentThemeFontSize}"/>