body {
  background-image: url('secret.png');
  background-repeat: repeat;
  background-size: auto;
  background-position: top left;
}

body::before {
  content: "";
  position: fixed;
  top: 0; left: 0;
  width: 100%; height: 100%;
  background-image: url('crt.png');
  background-repeat: repeat;
  background-size: auto;
  pointer-events: none;
  z-index: 9999;
  opacity: 0.2; /* tweak for intensity */
}