Trying to understand the overwriting of Garland's style.css file a bit better

I've created a custom theme based on Garland and I'd like to know which file is responsible for overwriting the style.css value and what rules are used. Some color overwrites seem to be a percentage of colors chosen on the theme configurator, etc. I'd like to also have a little more control over which values are overwritten and which are left alone. Can anyone get me pointed in the right direction?