You can use Unsettings to change the themes for GTK, window manger, icons and cursors. But it doesn’t support the installation of new themes.
With Unsettings you can switch off the global menu and the overlay scrollbars used in Unity.
In addition to just applying the settings Unsettings lets you to save your settings into and load from a text file (JSON format) so you can easily backup you settings or copy them to a different account.
Install Unsettings in ubuntu 12.04/11.10
Open the terminal and run the following commands
sudo add-apt-repository ppa:diesch/testing
sudo apt-get update
sudo apt-get install unsettings