[TxMt] Customized syntax coloring via Scope Selectors

I'm new to TextMate and am already recommending it to my students, but I don't understand the conventions for Scope Selectors.

Let's say I want to color all div tags in HTML red. I've tried variations on "entity.tag.div" in Prefs > Fonts & Colors without success. I've a basic familiarity with RegExp, and it looks like Bundle grammar supports RegExp, but I can't see where syntax coloring might be defined in the HTML Bundle either.

(For the record, I also can't figure out how to duplicate a Theme--say, if I want to tweak a version of the default Theme but keep the original too.)

Thanks for any help getting me going in the right direction.

jon