Hey Lorren,
Here’s what documentation there is on themes:
http://wiki.macrabbit.com/index/SyntaxThemes/
And here’s some info on the advanced selectors that you can use:
http://wiki.macrabbit.com/index/Selectors/
Otherwise you’ll need to pop open the syntax inspector window (Actions->Show Syntax Inspector) and explore the zones in an example document.
Sometimes the best way to go is to look at an example theme that you like to see how they did things. If you look at my themes, Earthworm is the cleanest, while Quiet Light is the most thorough.
One thing to watch out for is that sometimes colors don’t stick in places you think they should. This is typically caused by a placeholder theme for the language (PHP’s is particularly problematic), and usually the way to override the placeholder is to write a more specific selector. This is why you’ll see a bunch of duplicated things in Quiet Light, for instance. Theoretically, specifying the placeholder as an override at the top of the file is enough, but that rarely seems to work out.
Let us know if you have any other questions!