Respect background/border in ItemsControl.

Fixes #3431

src/Avalonia.Themes.Default/ItemsControl.xaml

@@ -1,10 +1,15 @@
 <Style xmlns="https://github.com/avaloniaui" Selector="ItemsControl">
   <Setter Property="Template">
     <ControlTemplate>
-      <ItemsPresenter Name="PART_ItemsPresenter"
-                      Items="{TemplateBinding Items}"
-                      ItemsPanel="{TemplateBinding ItemsPanel}"
-                      ItemTemplate="{TemplateBinding ItemTemplate}"/>
+      <Border Background="{TemplateBinding Background}"
+              BorderBrush="{TemplateBinding BorderBrush}"
+              BorderThickness="{TemplateBinding BorderThickness}"
+              Padding="{TemplateBinding Padding}">
+        <ItemsPresenter Name="PART_ItemsPresenter"
+                        Items="{TemplateBinding Items}"
+                        ItemsPanel="{TemplateBinding ItemsPanel}"
+                        ItemTemplate="{TemplateBinding ItemTemplate}"/>
+      </Border>
     </ControlTemplate>
   </Setter>
 </Style>