Load those styles all async like!

loadstyles.js

// TODO: convert into promise =)

// Load Style
function loadStyle(url, callback) {
  if (!url || url === '') return;

  var linkElement = document.createElement('link');
  linkElement.href = url;
  linkElement.setAttribute('rel', 'stylesheet');

  if(typeof callback === 'function') {
    linkElement.onload = callback;
  }

  document.head.insertAdjacentElement('beforeend', linkElement);
}

// Load Styles
/*
 * Expects array of objects in the format:
 * {
 *   url: 'path/to/style.css',
 *   callback: function () {}
 * }
 *
 * The callback is optional.
 *
 */
function loadStyles(styles, complete) {
  if (styles.length > 0) {
    styles.forEach(function(style) {
      loadStyle(style.url, style.callback);
    });

    if (typeof complete === 'function') {
      complete();
    }
  }
}