This seems like a really fun idea! A simple way of implementing it would be to offer the user a style to set some defaults and, after selecting it, allow them to answer a few simple questions on a form in order to customise it. All pages then (except, say, user's home nodes)could have a link tag to the stylesheet in the viewer's home node. Anonymous monks would, of course, require a default.

A home node, however, should be viewed in the owner's stylesheet though perhaps giving an option to switch would be fun ;)