http://www.csszengarden.com is a site for true web design geeks. CSS, because it is object oriented, is like a layered onion. The more you understand, the more you think, "oh wow!" UI changes become very easy because it separates the UI from the HTML data. You change a single character of .css code and, if you architected your HTML class definitions correctly, it'll update the look of all your HTML pages.
For anyone who hasn't visited zen garden, look for the "download the sample html file and css file" links. Each design uses the same (and extremely simple) HTML, but note how different the css files are. My most recent discovery: replacing the DIV tag's image background style without the use of javascript.