body {
  background-image: url("chrome://global/skin/media/imagedoc-darknoise.png");
  margin: 0;
}

body > :first-child {
  display: block;
  position: absolute;
  margin: auto;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  background: hsl(0,0%,90%) url("chrome://global/skin/media/imagedoc-lightnoise.png");
  color: #222;
}