Add missing background to the popup controls

src/Avalonia.Themes.Default/AutoCompleteBox.xaml

@@ -21,7 +21,8 @@
                  MaxHeight="{TemplateBinding MaxDropDownHeight}"
                  PlacementTarget="{TemplateBinding}"
                  IsLightDismissEnabled="True">
-            <Border BorderBrush="{DynamicResource ThemeBorderMidBrush}"
+            <Border Background="{DynamicResource ThemeBackgroundBrush}"
+                    BorderBrush="{DynamicResource ThemeBorderMidBrush}"
                     BorderThickness="1">
               <ListBox Name="PART_SelectingItemsControl"
                        BorderThickness="0"

src/Avalonia.Themes.Default/ComboBox.xaml

@@ -69,7 +69,8 @@
                    MaxHeight="{TemplateBinding MaxDropDownHeight}"
                    PlacementTarget="{TemplateBinding}"
                    IsLightDismissEnabled="True">
-              <Border BorderBrush="{DynamicResource ThemeBorderMidBrush}"
+              <Border Background="{DynamicResource ThemeBackgroundBrush}"
+                      BorderBrush="{DynamicResource ThemeBorderMidBrush}"                      
                       BorderThickness="1">
                 <ScrollViewer HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}"
                               VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}">

src/Avalonia.Themes.Default/ContextMenu.xaml

@@ -1,4 +1,5 @@
 <Style xmlns="https://github.com/avaloniaui" Selector="ContextMenu">
+    <Setter Property="Background" Value="{DynamicResource ThemeBackgroundBrush}"/>
     <Setter Property="BorderBrush" Value="{DynamicResource ThemeBorderMidBrush}"/>
     <Setter Property="BorderThickness" Value="1"/>
     <Setter Property="Padding" Value="4,2"/>

src/Avalonia.Themes.Default/MenuItem.xaml

@@ -62,7 +62,7 @@
                    PlacementMode="Right"
                    IsLightDismissEnabled="False"
                    IsOpen="{TemplateBinding IsSubMenuOpen, Mode=TwoWay}">
-              <Border Background="{TemplateBinding Background}"
+              <Border Background="{DynamicResource ThemeBackgroundBrush}"
                       BorderBrush="{DynamicResource ThemeBorderMidBrush}"
                       BorderThickness="{TemplateBinding BorderThickness}">
                 <ScrollViewer Classes="menuscroller">                  
@@ -113,7 +113,7 @@
                    IsLightDismissEnabled="True"
                    OverlayInputPassThroughElement="{Binding $parent[Menu]}"
                    IsOpen="{TemplateBinding IsSubMenuOpen, Mode=TwoWay}">
-              <Border Background="{TemplateBinding Background}"
+              <Border Background="{DynamicResource ThemeBackgroundBrush}"
                       BorderBrush="{DynamicResource ThemeBorderMidBrush}"
                       BorderThickness="{TemplateBinding BorderThickness}">
                 <ScrollViewer Classes="menuscroller">