<!DOCTYPE html>
<html>
<head>
<title>...</title>
<style type="text/css">
html, body {
margin: 0;
padding: 0;
height: 100%;
background-image: url('http://sites.google.com/site/jaimemontoya/Sample07.jpg');
}
.shadow {
position: fixed;
width: 100%;
height: 100%;
background-color: #000;
opacity: 0.7;
}
.center_block {
position: fixed;
left: 50%;
top: 50%;
margin: -100px 0 0 -100px;
width: 200px;
height: 200px;
background-color: #f00;
}
</style>
</head>
<body>
<div class="shadow"></div>
<div class="center_block"></div>
</body>
</html>