Our CSS is located at main.scss and gets processed through Sass.
To generate the CSS during development after any changes to that file, you must run:
npm run sass
which generates the final CSS file:
You then need to explicitly include that main.css file in your --template. For example, our main.liquid.html contains a line:
<link rel="stylesheet" type="text/css" href="{{ root_relpath }}main.css">
where root_relpath is explained under Section "--template".