A while back when WordPress came out with their latest blog software version 3.0, along with a pretty decent vanilla default theme called Twenty-Ten, I wrote a long post detailing some of my attempts to modify and customize the theme. I think initially they didn't really want people mucking with it, or maybe they just forgot to mention and fully document the proper way to do it — but whatever the case, I found some of the places in the main theme files to change color, font sizes, heading styles and quite a few other details. I loved the basic functionality and layout of Twenty-Ten, but I didn't at all like their typographic and color choices.
This post led to more traffic to this site than my various vegetarian food recipies (esp the Chicken Dijonaise one), but the spirit of accuracy in documentation behooves me to mention that the correct way to modify any downloaded theme is to use Child Theme overrides (documented here in the official WordPress Codex).
Why 'child theme' overrides instead of just modifying the theme itself? Because if at any future date there is an update to the theme and you download and install it, the defaults may overwrite any changes you've made. It has the same overall effect as modifying the theme files directly, but in a way it's like the difference between adding an accessory to your car versus having a bunch of custom body- and frame-work done to it.
(For what it's worth, I have updated WordPress to its latest version twice (now at 3.0.3) and fortunately it hasn't overwritten my theme overrides…which I keep meaning to redo in the proper Child Theme format, but never get around to it).
Anyway, if you're interested in some esoteric details, and some hints as to how one might go about coding those Child Theme overrides, feel free to check out that older post, but for now I've turned off comments in it. Partly because I think some of the drive-by commenters of late still aren't grokking that the basic premise of the original post is no longer a practice I'd recommend, and partly because unregistered commenters have their comments stuck in my moderation queue for weeks at a time and I didn't want folks to think I was deliberately ignoring them.