What you are looking to do is done by having a couple of themes on the server (theme1.css, theme2.css, theme3.css, etc.) and letting the user select the theme he likes. You can then save in the database with the user profile the theme the user chose (theme2.css). When the user then displays his page, you include at the top of the page the theme2.css instead of the theme default.css.
Hope it gives you an overview.