Do not override default (null) popup background

src/Avalonia.Themes.Default/OverlayPopupHost.xaml

@@ -1,7 +1,6 @@
 <Style xmlns="https://github.com/avaloniaui" 
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Selector="OverlayPopupHost">
-  <Setter Property="Background" Value="{DynamicResource ThemeBackgroundBrush}"/>
   <Setter Property="Foreground" Value="{DynamicResource ThemeForegroundBrush}"/>
   <Setter Property="FontSize" Value="{DynamicResource FontSizeNormal}"/>
   <Setter Property="FontFamily" Value="{x:Static FontFamily.Default}" />

src/Avalonia.Themes.Default/PopupRoot.xaml

@@ -1,7 +1,6 @@
 <Style xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Selector="PopupRoot">
-  <Setter Property="Background" Value="{DynamicResource ThemeBackgroundBrush}"/>
   <Setter Property="Foreground" Value="{DynamicResource ThemeForegroundBrush}"/>
   <Setter Property="FontSize" Value="{DynamicResource FontSizeNormal}"/>
   <Setter Property="FontFamily" Value="{x:Static FontFamily.Default}" />

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

@@ -1,5 +1,4 @@
 <Style xmlns="https://github.com/avaloniaui" Selector="OverlayPopupHost">
-  <Setter Property="Background" Value="{DynamicResource SystemControlBackgroundAltHighBrush}"/>
   <Setter Property="Foreground" Value="{DynamicResource SystemControlForegroundBaseHighBrush}"/>
   <Setter Property="FontSize" Value="{DynamicResource ControlContentThemeFontSize}"/>
   <Setter Property="FontFamily" Value="{DynamicResource ContentControlThemeFontFamily}" />

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

@@ -2,7 +2,6 @@
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
   <Style Selector="PopupRoot">
     <Setter Property="TransparencyLevelHint" Value="Transparent" />
-    <Setter Property="Background" Value="{x:Null}" />
     <Setter Property="Foreground" Value="{DynamicResource SystemControlForegroundBaseHighBrush}"/>
     <Setter Property="FontSize" Value="{DynamicResource ControlContentThemeFontSize}"/>
     <Setter Property="FontFamily" Value="{DynamicResource ContentControlThemeFontFamily}" />