?wptheme= Problems

I’m trying to use the ?wptheme= on my site, to see what different themes look like without changing between each one in the admin panel.

I don’t want to put a theme switcher on the site.. just a litte experimenting.

The site: http://www.trixdesigns.com

I have themes: red-train, flex, default, and a few others, but I figure with those exact names you should be able to make it work (if it works at all anymore)