Look what ckonze found over at Cheapassgamer: a Halo ODST premium theme for the xbox 360. Currently marked as Free for gold members. I’d recommend logging in and adding it to your download queue in case a price is added later.
http://marketplace.xbox.com/en-US/games/media/66acd000-77fe-1000-9115-d8024d530877/
Though, in reality, most if not all of these Premium Themes should be free. And the bottom part of the interface should be optionally transparent to allow for more custom themes and wallpapers.