app/app.html

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Hello Electron React!</title>
    <script>
      (function () {
        const font = (process.env.HOT ? '../' : './') +
          'node_modules/font-awesome/css/font-awesome.min.css';
        const link = document.createElement('link');
        link.rel = 'stylesheet';
        link.href = font;
        document.write(link.outerHTML);
      }());
    </script>

    <script>
      (function() {
        if (!process.env.HOT) {
          const link = document.createElement('link');
          link.rel = 'stylesheet';
          link.href = './style.css';
          document.write(link.outerHTML);
        }
      }());
    </script>
  </head>
  <body>
    <div id="root"></div>
    <script>
      {
        const script = document.createElement('script');
        const port = process.env.PORT || 3000;
        script.src = (process.env.HOT) ? `http://localhost:${port}/static/bundle.js` : './bundle.js';
        document.write(script.outerHTML);
      }
    </script>
  </body>
</html>