strapi/packages/strapi-helper-plugin/lib/internals/postcss/postcss.config.js

const postcssSmartImport = require('postcss-smart-import');
const precss = require('precss');
const autoprefixer = require('autoprefixer');

module.exports = {
  plugins: [
    postcssSmartImport(),
    precss(),
    autoprefixer(),
  ],
};