strapi/docs/css/sass.md

Can I use Sass with this boilerplate?

Yes, although we advise against it and do not support this. We selected PostCSS over Sass because its approach is more powerful: instead of trying to give a styling language programmatic abilities, it pulls logic and configuration out into JS where we believe those features belong.

As an alternative, consider installing a PostCSS plugin called PreCSS: it lets you use familiar syntax - $variables, nesting, mixins, etc. - but retain the advantages (speed, memory efficiency, extensibility, etc) of PostCSS.

If you really still want (or need) to use Sass then...

1. Change internals/webpack/webpack.base.babel.js so that line 22 reads

   test: /\.s?css$/,
   

   This means that both .scss and .css will be picked up by the compiler

2. Update each of

   • internals/webpack/webpack.dev.babel.js
   • internals/webpack/webpack.prod.babel.js

   changing the config option for cssLoaders to

   cssLoaders: 'style-loader!css-loader?modules&importLoaders=1&sourceMap!postcss-loader!sass-loader',
   

   Then run npm i -D sass-loader node-sass

...and you should be good to go!