Ubuntu is themed not from apt but from the Control Panel at System, Preferences,
Appearance, I think. I am in KDE 4.2 now, so can't check. You can download any
Gnome themes from the Gnome artwork site or from http://www.gnome-look.org/
You can view them before you install them and you can change them easily
afterwards if you aren't happy.

If you use Compiz or Emerald then you can find themes for that at their site.