The parsing into the css file should be easy, a user and newbie friendly GUI should be the harder part.
I'm not sure if i personal stylesheets are a good idea for unexperienced users. Most of them don't have any clue about "design" and probably a lot of them will send mails, cause they can't read anything... Black text on black background, you know... ;-)
I implemented such a functionality into an cms, but even the customers/site owners werent't talented enough to use it probably. Personal stylesheets can mess up the corporate identity of the site/forum very easily, so many companies/bigger sites don't want to give the desing into the hands of their visitor.
On the other hand, it would be a nice add-on for geeks. Maybe your generator can even be ported into the admin-area, since the editing features of the stylesheets aren't very comfortable at the moment.
