add postcss-custom-properties to create-react-app without "eject"

package.json

"scripts": {
  "postbuild": "postcss --replace build/**/*.css"
}

postcss.config.js

module.exports = ctx => ({
  map: ctx.options.map,
  replace: ctx.options.replace,
  plugins: {
    'postcss-custom-properties': { preserve: true },
  },
});