We will use SASS/SCSS for this. It'll make everything easier.

To install SASS you need rubygems and then, you simply do:

$ gem install sass

I'd recommend to keep a lot of sass and scss files in the css/ folder and include all of them using main.sass and the @import syntax. Example:

// to import the file "article.sass" or "article.sass" we use
@import "article"

To compile the SASS files into a CSS every time something is changed, we use:

(from the css/ folder)
$ sass --watch main.sass:styles.css

And it'll watch changes in every SASS and SCSS files and update the CSS output. We only include styles.css in our HTML. styles.css should not be commited into the repository.