Allow overriding inner menu padding.

src/Avalonia.Themes.Default/MenuItem.xaml

@@ -69,7 +69,6 @@
                                     Items="{TemplateBinding Items}"
                                     ItemsPanel="{TemplateBinding ItemsPanel}"
                                     ItemTemplate="{TemplateBinding ItemTemplate}"
-                                    Margin="4 2"
                                     Grid.IsSharedSizeScope="True"/>
                 </ScrollViewer>
               </Border>
@@ -119,7 +118,6 @@
                                   Items="{TemplateBinding Items}"
                                   ItemsPanel="{TemplateBinding ItemsPanel}"
                                   ItemTemplate="{TemplateBinding ItemTemplate}"
-                                  Margin="2"
                                   Grid.IsSharedSizeScope="True"/>
                 </ScrollViewer>
               </Border>
@@ -130,6 +128,10 @@
     </Setter>
   </Style>
 
+  <Style Selector="MenuItem /template/ ItemsPresenter#PART_ItemsPresenter">
+    <Setter Property="Margin" Value="2"/>
+  </Style>
+
   <Style Selector="MenuItem:selected /template/ Border#root">
     <Setter Property="Background" Value="{DynamicResource ThemeAccentBrush4}"/>
     <Setter Property="BorderBrush" Value="{DynamicResource ThemeAccentBrush}"/>