For in-depth docuemntation please read Compiling Assets (Laravel Mix).

To make changes to the CSS changes please use resources/themes/default/sass/app.scss.

webpack.mix.js contains the line to indicate which file should be used and where it should be outputted at.

mix.sass('resources/themes/default/sass/app.scss', 'public/default/css');

To compile SASS to CSS. You should run

npm run production