injectCSS(...files) {
    files.forEach((file) => {
      const data = fs.readFileSync(file);
      const styles = document.createElement('style');
      styles.innerHTML = data.toString();

      document.querySelector('head').appendChild(styles);

      debug('Append styles', styles);
    });
  }
Injects the contents of a CSS file into the current webview

@param {Array} files     CSS files that should be injected. This must
                         be an absolute path to the file