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.
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.
Press Ctrl+Alt+T to launch terminal, and type in this command to install gnome-tweak-tool (Advance Settings):
sudo apt-get install gnome-tweak-tool
Run this 3 commands to install user theme extension:
sudo add-apt-repository ppa:ferramroberto/gnome3
sudo apt-get update
sudo apt-get install gnome-shell-extensions
Now you need restart gnome shell (press Alt+F2, input r and hit enter) or log out and log back to enable user theme extension:
Installing Gnome Shell Themes:
Now you need to download some gnome shell themes (gnome-look.org).
Re-launch gnome tweak tool (Advance Settings), under Theme tab browse to install downloaded themes and choose one in drop-down menu.
It creates a “.themes” directory under your home folder and all themes installed there. You can manually place an extracted theme here and apply via gnome-tweak-tool.