Plugins / libraries for user-specific CSS?

Hi all,

I'm creating a social networking site which (obviously) will be mostly
user-created content. I'd like for users to also be able to have some
control over the look of the site - ie for them to choose 'skins' or
create a custom stylesheet overriding some of the basic CSS attributes
so they can choose their preferred font, etc. In an ideal world, I'd
also let them choose from a 'menu' of certain elements so that they
could specify a minimum font size, a colour scheme, etc and mix and
match rather than having to apply an 'off-the-peg' template.

I feel sure that I'm not the first person to want to allow some user
control over CSS in a Rails app, but I'm not turning up much in the
way of help via Google. Can anyone point me to any good tutorials /
plugins / libraries which might help me get started with this? I've
given my ideal scenario above just on the offchance that some perfect
solution exists, but really even a very basic example would be useful
- I don't want to reinvent the wheel if there are also some useful
models in existence.

Thanks in advance!