CobiBird theme is a lightweight theme, based on GreyBird. It is compatible with the latest GTK.

This is how you install Cobibird on Ubuntu 13.04 Raring Ringtail, Ubuntu 12.10 Quantal Quetzal, Ubuntu 12.04 Precise Pangolin, Linux Mint 14 Nadia and Linux Mint 13 Maya.
screenshot from gnome-look.org
Add the repo:
$ sudo add-apt-repository ppa:noobslab/themes
Update the repo index:
$ sudo apt-get update
Install the theme:
$ sudo apt-get install cobibird