Add purgeCSS to the list of style loaders

webpack.config.js

{
  loader: require.resolve('postcss-loader'),
  options: {
    // Necessary for external CSS imports to work
    // https://github.com/facebook/create-react-app/issues/2677
    ident: 'postcss',
    plugins: () => [
      require('postcss-flexbugs-fixes'),
      require('postcss-preset-env')({
        autoprefixer: {
          flexbox: 'no-2009',
        },
        stage: 3,
      }),
      postcssNormalize(),
      purgecss, // <---- THIS PART IS NEW
    ],
    sourceMap: isEnvProduction && shouldUseSourceMap,
  },
},