Looks like some pretty solid CSS with one exception: you are changing font size and colors for the default figure tag. While it's not a common tag, it is used elsewhere. This may cause issues for some users, causing them to have to prefix your selectors.
Yes, in addition to what javajosh found, your class names are rather generic and I imagine might lead to clashes. I'd personally recommend prefixing your class names with something (rather like FontAwesome now prefixes their classnames with 'fa-').
Maybe the figure element is so rare that this doesn't matter, but that is making some sort of an assumption.
Apart from that this looks great, and I could definitely see myself using this in a project I'm working on!
Oh, sorry... one other thing. I just noticed in the README you redirect people to the website for documentation. It's fine to link to the website of course, but I'd recommend putting at least some usage examples in the README itself so people landing there get some idea of what the project is about without having to navigate away (which might turn some people off).
