Depending on what exactly you mean by "skinning", you could simply have a number of different CSS stylesheet files, then when a user selects a "skin", save their choice in a cookie. Check for that cookie on each page load, and if there, use its value to specify which stylesheet file to use. If it's a site with some sort of login and user database, you'd probably save their choice in the database instead, then once logged in you might keep the choice in a session variable so that you don't have to grab it from the DB on each page.
