Hi Mark,
The Backstage opens in an adorner layer that is defined within the RibbonWindow. That way, it's able to cover the title bar UI of the RibbonWindow properly, just like in Office. I wouldn't recommend trying to inject margin space between the Ribbon and the containing UI since that goes against the ribbon design ideals.
Is there a specific reason you're trying to do that? Note that there are items in the Microsoft Ribbon requirements specification like this: "The Ribbon MUST extend the full width of the application window at all times." Using margins around the ribbon would violate that licensing requirement by Microsoft for use of the Ribbon UI.
