This is my page on CSS. Really, there isn't that much to cover in CSS. The basic elements are quite simple - but there are still some concepts that are useful to keep in mind when writing clean code. Style is particularly important when writing CSS, as it is easy to get out of hand nesting weird selectors on top of each other.
CSS can really only be learned through experience, as you start to build up a whole toolkit of tricks for doing certain things. For instance, it's not obvious that
margin: 0 auto would horizontally align something.