How about a theme switcher ? Its auto and require single javascript include and editing an select box, thats it. Have a look at this,
dynamicdrive.com/dynamicindex9/styl
...
Its really tiring to browse through all designs, read commnts, test the pages, and comment again, you get the feeling before reaching even 10th design.
I suggest using it, coding by hand a bit to customise the select box for fast themes/design name and url, probably need to remind user to reload/refresh for the icons, and that it. Fast theme switching on the fly (icons might need refresh though).