Ambiance Theme gone ! i've left with radiance , when i try to select it i got a metacity like theme.(tried to reinstall ambiance themes etc without any luck. I'm using ubuntu 12.10 64bit.
open synaptic package manager and type light-theme and mark them for reinstaling it will fix yo
Boot from ubuntu live cd and copy themes folder from filesystem( usr/share/..to any flash drive.Then replace by this themes folder in your system.Install common light-theme pachages via synaptic or at least try downloading this theme http://www.webupd8.org/2011/12/light-themes-evolved-ambiance-and.html chose it via gnome tweak tool