Edit in JSFiddle

<body>
  <div class="wrapper">
    <div class="modal">
      <div class="blurrer"></div>
      <div class="message">
        <div class="row">
          I am a transparent blurry modal box which can adapt to any screen size, I'm very cool!
        </div>
      </div>
     </div>
  </div>
</body>