Orion, a cool, light GTK3 theme pack is now available for GNOME 3.4 (which includes Ubuntu 12.04 Precise Pangolin) and is compatible with the latest Unico GTK engine. The new release also brings support for Openbox.

Orion includes GTK2/3, XFCE and Openbox themes and is fully compatible with GNOME Shell (though it doesn't include a GNOME Shell theme, but you can use Elegance for instance, by the same author) and Unity.

Install Orion GTK3 theme