Once again, installing Gnome Shell themes has changed a tad. Below are the instructions for installing Gnome Shell theme in Ubuntu 11.10.
1. If you don’t have it already, install the Gnome Tweak Tool:
sudo apt-get install gnome-tweak-tool
2. Install the User-Theme GNOME Shell extension from PPA.
sudo add-apt-repository ppa:ferramroberto/gnome3
sudo apt-get update
sudo apt-get install gnome-shell-extensions-user-theme
3. Once both of the above are installed, restart Gnome Shell. Either hit Alt+F2 and run r, or log out and log back in.
4. Open the Gnome Tweak Tool. Under Shell extensions, make sure the User Themes Extension is toggled on.
5. Under Themes, browse for a theme you downloaded*.
*Note: You can download Gnome Shell themes here.
6. Choose your theme from the dropdown menu.