Source: http://www.developphp.com/view.php?tid=1037

Learn how to allow visitors to change the themes of your website using CSS Style Sheets and PHP Cookies.

It uses PHP Cookies and well structured CSS style folders and style sheets to allow any visitors to theme your website according to layouts you have made.

If your style sheet is set up smart your buttons, background, backgrounds for tables and divs, size of things, placement of things, and everything can be changed by altering each theme's style sheet.

Using PHP cookies we can store their theme choice in a cookie file inside their browser software so even when they close their browser down, the next time they come back their theme is in place as long as the cookie is set to last. If the user deletes all cookies from within their browser options, the theme will revert back to your original site theme.