Sorry, there isn't really any good documentation on the CSS apart from what you might find at this site. I'll try to write something up fairly soon; it's been on my list for, uh, a while now. In this connection, we want to try to simplify the CSS, hopefully for Tiki 8. This is in part due to the new "theme generator" feature, which would benefit from fewer colors per theme for example (nearly the same colors could be consolidated in some cases, etc.). And I want to reduce the redundancy of things getting specified in multiple CSS files, where possible. Really, some rethinking from the bottom up is needed, which Jonny and I have talked a little about (and probably many people have thought about/hoped for ;-) ).