Joomla Function

Simple Joomla function I want a Front-End function to allow a user to change the Joomla template used for the current site. So that they can see what their site would look like with a different template. This needs to be a simple drop list. The data for the drop list can come from a MySql Table or a simple TEXT file that the Admin can change. (If MySQL table (preffered) - must include a form for updating the table).