It's almost certainly the case that the newer visual style is something that needs to be opted in to, and Notepad just hasn't been updated for so long that it's using the older code still.
You might find that applying the Classic style to your application (there are some tutorials on how to "force" WPF into using a specific style regardless of the active Windows style) does the trick. Otherwise, you'll need to develop the style yourself by retemplating MenuItem and StatusBar.
Controls for WPF, Windows Forms and Silverlight at
http://www.divelements.co.uk