Have it possible where people can upload to a new folder called 'themes' where you create a folder of your choosing, upload a CSS file and preview file then on the config for the site they choose from a list of names of styles in that folder which the names would have to be kept in a PHP or XML file. A theme manager would be nice on top of this allowing us to see what each theme looks like but if people provide a screenshot with their download we wouldn't need a theme manager.