I really have to recommend LESS CSS. at
http://lesscss.org/. I include a link because searching for "less" with Google is pretty much impossible if you don't know what other keywords to use.
LESS extends CSS with dynamic behavior such as variables, mixins, operations and functions.
I use WinLess by Mark Lagendijk, which automatically compiles the Less script into CSS every time I save the Less file.
An example of why it's so important? Well, I use the variables, list them at the top of my script, like:
@body-bg: #C9A788;
@dark: #7F5B3A;
@mediumdark: #815C3A;
@medium: #B08053;
Then I use the variables in the CSS instead of the HTML color, and when I want to change the color scheme of the theme, I don't have to search through the whole Style sheet to find 1000+ instances that need to be changed. I just change the stuff at the top, recompile, and I'm good to go.
I only discovered it a couple of weeks ago, but wish I had discovered it much sooner. It has saved me a lot of time. I plan to convert all my CSS into Less.