There have been proposed quite a few themes for Ubuntu Karmic Koala 9.10 and if you want to install them all in Ubuntu Jaunty for instance, you can just do:
sudo apt-get install community-themes
But not all the themes will be installed because the community-themes package in Ubuntu Jaunty repositories is currently version 0.14 I believe, while the Karmic version from Launchpad is already 0.17. But you can install the Ubuntu Karmic community-themes package (currently version 0.17) in Jaunty without any issues (in fact, it might work on Gutsy, Intrepid, etc, but I cannot test it). Obviously, if you use Ubuntu Karmic Koala, this will also work for you. Read on!