If you start with Tabula Rasa, http://www.dreamwidth.org/doc/s2/ links to some good articles about its CSS structure. I've also made my own little memo here.
As timeasmymeasure said, the longest part is figuring out what each part does and how it's called. :)

About mood themes, I haven't seen a community for custom ones so maybe you could create it! Also to upload any you've created you can go to http://www.dreamwidth.org/manage/moodthemes but I think this is reserved to paid users.