Gnome Shell is customization is awesome, you can theme every part of your desktop from applet menus to panel every thing is themeable. Even on-screen keyboard can be customized through themes.
If you are using Ubuntu 12.04 Precise Pangolin you can see our previous article Install Gnome-Shell in Ubuntu 12.04. And for icon themes you should refer to AwOken Icon Theme or Faenza Icon Theme as they are better supported in Ubuntu. To install themes see Install GNOME-Shell themes on Ubuntu.
Here is our list of top 5 Gnome-Shell themes which can be installed on Ubuntu, Fedora, Arch and other Linux distributions. All of the following themes are Gnome 3.4 compatible.