I am currently in the process of working on developing a theme for this site. This will tell me what I need to know to theme other drupal sites. Consequently, the site looks a little whacked out right now. Some things may be a little hard to read; if you have problems, send me mail.
Drupal themes are pretty much all CSS. This makes good sense, because you can't do a lot of things without CSS - so you might as well use it. My current theme is based on the box_grey theme, which uses tables to prevent malformed content from running away and breaking the page layout. This actually presents some challenges, because the Table DOM doesn't support a lot of properties that I like to mess with, but everything else and its brother supports them; things like separate border properties for the four sides, for example. (Tables have border properties, sure, but table cells?)
If you want to edit drupal themes, you owe it to yourself to get the Web Developer Extension for Firefox. In particular, it allows you to edit CSS. Don't change to another tab, though, because it will lose your edits. Open reference pages in another browser window instead of a tab. It also shows you tables, image attributes, javascript, et cetera.