2
if you mean just switch from one theme to another then you need the theme switcher block turned on. (its in "system") and basically for you use only
if you mean you want a differnt theme for different parts of your site then you need the rather excellent "defacer" module
http://xoops.svn.sourceforge.net/viewvc/xoops/XoopsModules/defacer/
Someone else might know something different
hth
phil