SCSS

The Shopware 6 storefront uses SCSS code for styling (unlike Shopware 5 which used LESS).

Sass is the most mature, stable, and powerful professional grade CSS extension language in the world.

– sass-lang.com

SCSS files can be supplied with themes or plugins to the compiler, which compiles all the SCSS code of the storefront into the "all.css".

Further information

Sass: Syntactically Awesome Style Sheets

CSS with superpowers