This site is now using SCSS stylesheets. SCSS extends CSS3 syntax and is converted to CSS before the site is published. Read more on SCSS on its homepage.
Each SCSS file is named exactly as the generated CSS file:
default.scss ⇒ default.css
sass --style compressed style:style
sass --style compressed --watch style:style
This regenerates the CSS each time a SCSS file is changed. The compression style can be omitted or changed here.