Theme tutorials or how-tos?

I have been working on modifying and creating my own Drupal themes. I have found quite a bit about CSS online, and have been able to make minor modifications to fonts, backgrounds, etc. in my themename.css file, but I think what I really need is more detailed, drupal-specific information. In particular, I'm looking for a little explanation of the sections of the layout.css and themename.css files that are not obviously self-explanatory to people not very familiar with Drupal. Also more information on site layout and how it is controlled would be useful. Does anyone know where I could find more on these topics? I had a lot of success with installation, modules, etc., but the Drupal handbook and Google searches have not been very helpful for this type of stuff. Thanks for any suggestions you may have.