CSS Overlay Techniques

<html>
  <body>
    <div class="overlay"></div>
  <body>
<html>

html, body {
  min-height: 100%;
}

body {
  position: relative;
}

.overlay {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  z-index: 10;
  background-color: rgba(0,0,0,0.5); /*dim the background*/
}