[_ck_]

My theme switcher plugin is capable of doing this quite easily, it already allows each visitor to select the theme they want.

All you would need to do is change the cookie support code into session support code so the server controls which theme is seen instead of the visitor.

Should take all of an hour to convert for someone who understands cookies and sessions.

Actually if the domain is fixed you can just use $_SERVER['HTTP_HOST'] or $_SERVER['SERVER_NAME'] to determine the theme to use which makes it even easier.