Rather than modify the original css files, I've found it easier to add my won css file to the bottom of the cascade and then override styles there. It's not 100% simple, though, because 1) I sometimes have to use !important even when it doesn't seem like should have to and 2) the css is a bit spaghetti so results are occasionally unexpected. I do hope to win back this grief and more when I never have to merge CSS on upgrades.
