Help adding css changer

I want to let the admin of my app be able to change the css used for the
app from within the admin panel, what I want is a dropdown box with a
list of stylesheets, so when it gets submitted, the stylesheet to be
used is changed site-wide.

I think its best to do this via a database table but I am unsure of how
to go about storing the data as well as implementing a dropdown box.

If anyone could help I would be most grateful!