We have built the theme using SCSS. The .scss files are included within the theme package, so should you want or need to make any changes to the theme, we strongly suggest you make use of SCSS to do so.
SCSS is a meta-language on top of CSS that’s used to describe the style of a document cleanly and structurally, with more power than flat CSS allows. SCSS both provides a simpler, more elegant syntax for CSS and implements various features that are useful for creating manageable stylesheets.
You can find out more about SCSS here.