Update menu item default themes.

src/Avalonia.Themes.Default/MenuItem.xaml

@@ -139,4 +139,4 @@
   <Style Selector="MenuItem:disabled">
     <Setter Property="Opacity" Value="{DynamicResource ThemeDisabledOpacity}"/>
   </Style>
-</Styles>
+</Styles>

src/Avalonia.Themes.Default/Separator.xaml

@@ -1,6 +1,7 @@
 <Styles xmlns="https://github.com/avaloniaui">
 
   <Style Selector="Separator">
+    <Setter Property="Focusable" Value="False"/>
     <Setter Property="Template">
       <ControlTemplate>
         <Border BorderBrush="{TemplateBinding BorderBrush}"
@@ -22,4 +23,4 @@
     <Setter Property="Height" Value="1"/>
   </Style>
 
-</Styles>
+</Styles>