title: CSS animate transition of background image getting darker
body: <p>So I'm building a page that has a background image:</p>
  
  <pre><code>body {
      background:url(images/bg.png) center center no-repeat fixed;
      -webkit-background-size:cover;
      -moz-background-size:cover;
      -o-background-size:cover;
      background-size:cover
  }
  </code></pre>
  
  <p>The page loads with a div that looks like a alert/message. When this div loads I want a short animation of the background-image getting darker,<br>
  Right now I use this:</p>
  
  <pre><code>back { 
      background:rgba(0,0,0,.95);
      display:block;
      height:100%;
      left:0;
      opacity:1;
      position:fixed;
      top:0;
      width:100%
  }
  </code></pre>
  
  <p>This makes it darker, but I want this to happen gradually in a smooth transition/animation.</p>