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
To install Orion in Ubuntu 12.04 or 11.10, you can use Satya's themes PPA:
sudo add-apt-repository ppa:satyajit-happy/themes sudo apt-get update sudo apt-get install orion-gtk-theme
For other Linux distributions, download Orion GTK3 theme for GTK 3.4 via DeviantArt.