I don't think the full theming engine (with xml and whatever) should be part of Haiku; maybe you already know of the BeTheme project. Still, Haiku should support replacing the looks and feels, even if it does only provide a single version in the stock install.
The "theming engine" would be a stand-alone preflet. Whether or not Haiku includes it is another matter.
However, the app_server needs to be able to support swapping out the control look and decorators at a minimum, and ideally also the window behavior. The remainder is already easily performed with the current APIs...though you may end up with redundant preflets...
--The loon
