Adding user ability to customize their frontend views

I am working on a quiz application that has an administration panel where client should be able to upload their own frontend styling for their quizes. Instead of adding many fields for all possible values and configurations, it would be good if the client could upload custom templates, much like you do in MailChimp or Campaign Monitor.

Are there anyone with experience in implementing this kind of functionality. What is the best way to allow a user to edit the look and feel of their quiz?