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
CSS with superpowers