You don't need any help to turn off your themes!
Just go to your themed public site - if you're signed in, this link should take you there:http://www.xanga.com/home2.aspx
Go ahead and click on the "Edit this page" link at the top of the page. See the checkbox on the left side that says, "make this your active theme"? Just uncheck that, and hit save.
Now your theme should no longer be active!