You will have to add third party PPA launchpad repo to your
synaptic package manager to download these themes.
To Add the PPA launchpad repo, Goto
System->Administration->Software Sources, goto the Third-
Party Software tab and Click on ADD.
Copy and paste the new PPA Launchpad repo address, corresponding to
the version of Ubuntu you are using. Given below are the PPA for
Ubuntu jaunty, Intrepid and Hardy respectively."