Add missing background to the presenters and popup roots

src/Avalonia.Themes.Default/FlyoutPresenter.xaml

@@ -2,7 +2,7 @@
   <Style Selector="FlyoutPresenter">
     <Setter Property="HorizontalContentAlignment" Value="Stretch" />
     <Setter Property="VerticalContentAlignment" Value="Stretch" />
-    <Setter Property="Background" Value="Transparent" />
+    <Setter Property="Background" Value="{DynamicResource ThemeBackgroundBrush}"/>
     <Setter Property="BorderBrush" Value="{DynamicResource ThemeBorderMidBrush}" />
     <Setter Property="BorderThickness" Value="1" />
     <Setter Property="Padding" Value="4" />

src/Avalonia.Themes.Default/MenuFlyoutPresenter.xaml

@@ -1,6 +1,6 @@
 <Styles xmlns="https://github.com/avaloniaui">
   <Style Selector="MenuFlyoutPresenter">
-    <Setter Property="Background" Value="Transparent" />
+    <Setter Property="Background" Value="{DynamicResource ThemeBackgroundBrush}"/>
     <Setter Property="BorderBrush" Value="{DynamicResource ThemeBorderMidBrush}" />
     <Setter Property="BorderThickness" Value="1" />
     <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto" />

src/Avalonia.Themes.Default/PopupRoot.xaml

@@ -1,6 +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="Foreground" Value="{DynamicResource ThemeForegroundBrush}"/>
   <Setter Property="FontSize" Value="{DynamicResource FontSizeNormal}"/>
   <Setter Property="FontFamily" Value="{x:Static FontFamily.Default}" />

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

@@ -1,6 +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="TransparencyLevelHint" Value="Transparent" />
     <Setter Property="Foreground" Value="{DynamicResource SystemControlForegroundBaseHighBrush}"/>
     <Setter Property="FontSize" Value="{DynamicResource ControlContentThemeFontSize}"/>