You have probably heard about SASS. If not, click here, and read about it.
Once you read about it you will probably think "I'll try that on my next site."
Don't do that. Covert your current project to SASS right now. Well, not SASS, SCSS.
Unlike SASS, SCSS accepts valid CSS as input so you can use your current *.css files by just renaming them to *.scss.
Well, there is a little more to it than that, but not much more.