docsify/build/mincss.js

import cssnano from 'cssnano';
import path from 'path';
import fs from 'fs';

const files = fs.readdirSync(path.resolve('lib/themes'));

files.forEach(file => {
  file = path.resolve('lib/themes', file);
  cssnano
    .process(fs.readFileSync(file))
    .then(result => {
      fs.writeFileSync(file, result.css);
    })
    .catch(e => {
      console.error(e);
      process.exit(1);
    });
});