Not being flip or sarcastic. It's a huge, gnarly framework -- at least it looks that way at first. Lots of overlapping selectors, magical HTML rewrites, long-standing conventions (some of which are actually documented).
I'm sure it gets easier as you get familiar with it, but you have to look in bunches of files to find where things are changing. And you can't just look at one page and know what you're changing. Lots of selectors are in there for elements that only appear on-screen during very specific circumstances.
I'm at the point of learning a new framework. I want to understand Wordpress -- and in particular WP themes -- before I start overlaying another set of selectors.