Have you ever wondered which theme is more appropriate for your website? You probably hate this, because you have to pick one and drop the others, right? Well, this plugin will solve your dilemma. “Front-end Theme Changer” lets your users select the theme they like most from a dropdown (select) menu directly from the front-end.
Demo: http://codecanyon.net/item/frontend-theme-changer-for-wordpress/11269526