The only way that I could think of doing this is via a cookie which expires ... or by using sessions which keep track of the style in use (if the site has some sort of backend web dev language).
This site is based off of phpBB forum software, and there is actually a mod/hack available @ phpBB which allows non-registered guests to change the template based on a cookie. However, the hack is very phpBB specific. You might want to take a look at it to see how it's implemented, but I doubt it would be much help.
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