build.styles.js

const postcss = require('postcss');
const importCSS = require('postcss-import');
const copyCSS = require('postcss-copy');
const prefixCSS = require('autoprefixer');
const customPropertiesCSS = require('postcss-custom-properties');
const pxtoremCSS = require('postcss-pxtorem');
const pixremCSS  = require('pixrem');
// const cssnano = require('cssnano');
const fildes = require('fildes');
const readFile = fildes.readFile
const writeFile = fildes.writeFile;

const input = 'src/include.css';
const output = 'build/styles.css';

readFile(input)
.then(css => {
  return postcss([
    importCSS(),
    copyCSS({
      'src': ['node_modules/material-design-icons/iconfont'],
      'dest': 'build/fonts',
      'template': '[name].[hash].[ext]',
      'keepRelativePath': false
    }),
    prefixCSS({
      'browsers': [
        'android >= 4',
        'ios >= 8',
        'chrome >= 45',
        'firefox >= 3.6',
        'ie >= 8'
      ]
    }),
    customPropertiesCSS(),
    pxtoremCSS(),
    pixremCSS(),
    // cssnano({ 'autoprefixer': false })
  ])
  .process(css, {
    'from': input,
    'to': output,
    'map': {
      'inline': false
    }
  })
})
.then(result => {
  return Promise.all([
    readFile('src/index.html')
    .then(file => writeFile('build/index.html', file)),
    writeFile(output, result.css),
    writeFile(output + '.map', result.map)
  ]);
})
.catch(error => {
  console.log(error);
});