I think what is being asked here is the ability to change stylesheets each time the page this loaded. This is really Javascript stuff. If random, then you can randomly reference a different style sheet on page-load. If you want to do it systematically, you'll need to track it with a cookie.

The question remains, why would you want to do this? As a graphic designer by trade, I might point out that for identity and branding purposes it's best to stick to one look. Besides, it could just get "horsey" and confusing.

Consider selectable style sheets for font size, and accessibility issue. But that is an optional choice you give your visitors. My .02

Update: removed blockquotess