I Have a background image full screen, but i want a black overlay on it with a opacity of 0.7

This is my code:

```
body { 
  background:url(../images/bg.jpg) no-repeat center center fixed; 
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;}

```

How do i do this?

My html 

```
<!DOCTYPE html>
<!--[if lt IE 7 ]><html class="ie ie6" lang="en"> <![endif]-->
<!--[if IE 7 ]><html class="ie ie7" lang="en"> <![endif]-->
<!--[if IE 8 ]><html class="ie ie8" lang="en"> <![endif]-->
<!--[if (gte IE 9)|!(IE)]><!--><html lang="en"> <!--<![endif]-->
<head>

    <!-- Basic Page Needs
  ================================================== -->
    <meta charset="utf-8">
    <title>New Year Countdown</title>
    <meta name="description" content="">
    <meta name="author" content="">

    <!-- Mobile Specific Metas
  ================================================== -->
    <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1">

    <!-- CSS
  ================================================== -->
    <link rel="stylesheet" href="css/style.css">

    <!-- Favicon
    ================================================== -->
    <link rel="shortcut icon" href="images/favicon.ico">

</head>
<body>

<div id="over">k</div>

</body>
</html>

```

The over has to be an opacity overlay.