I always see websites where you can pick the theme, or color scheme of the website. How could I do that?

Like mysite.com/page.htm?theme=1

Not sure if thats what it would look like but whatever. And maybe It could save the theme to a cookie? Please help if you know how to do this.

Oh and i'm using html and css.

Not sure if its what you need but, try styleswitcher.js from this site http://alistapart.com/stories/alternate/