With windows and Mac versions available I can see theme support maybe one day. But as it stand it would appear you need to read up more on the changes to gnome in gnome 3 and and the use of compiz as the window manager in Ubuntu. Theme support is still very much deeply wound in all the interface code. The only exception being unity currently is not very extensible.
May I ask what themes you have tried? As all themes I have tried from deviant art or gnome-look work just fine.