I think I understand the principal of this, and if so it's exactly what I want, but I don't even begin to understand how to implement it.
Basically, some of my users hate the default Georgia font (although I love it), so I would love to give members the option to choose their default display font - i.e. not setting default options for how they post, rather choosing a font that posts are displayed in.
Can this be done? At the moment, the only way I can think of doing it is to duplicate my current Style, and set one to use serif fonts and the other to use sans-serif, then users can choose whichever they prefer. However, this is a very clunky way of doing it, and I'm inevitably going to have some people moaning that I've chosen the wrong fonts!