body::before {
  content: "";
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background: url('img/scanlines.png'); /* path to your PNG */
  pointer-events: none; /* so clicks go through */
  z-index: 9999;
  opacity: 0.1; /* adjust strength of effect */
}