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.
cscgal
The Queen of DaniWeb
19,433 posts since Feb 2002
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Like a cscgal said, set up a cookie. I'm assuming that you don't have an application server, just a regular web server like Apache. You can make a cookie in JavaScript then read it and set up whichever style they choose.
samaru
a.k.a inscissor
1,256 posts since Feb 2002
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