Johan Dahlin suggested GTK+ theming integration as a project on IRC, and I agree that this would probably be about the right level of difficulty/work. Or rather, supporting theming in our widget toolkit, even if the actual GTK+ bridge wasn't written.
IMO, such "themeing" should be done via a suitable implementation of the UXTHEME dlls (and related functionality) with the GTK+ theming integration being implemented as a "theme" file (AFAIK a theme file is just a windows DLL file with special stiff in it).
Although since doing it The Right Way would probobly require SxS/Activation Context/etc support in WINE, it might be harder :)