Ubuntu Guide shows you how to install Gnome Shell Themes in Ubuntu 12.04

After installed gnome-shell session on Ubuntu 12.04, you may be interested to try out a few themes on your desktop, and this simple tutorial will show you how to do it.
Preparation:

Apart from ensuring you actually have gnome-shell installed, the first thing need to do is installing gnome-tweak-tool and gnome-shell-extensions-user-theme.

  How to Install Gnome Shell Themes in Ubuntu 12.04