div#content {
  background-size: cover;
}
body.light div#content {
  background: url('/img/bg_light.jpg') no-repeat center center fixed;
}
body.dark div#content {
  background: url('/img/bg_dark.jpg') no-repeat center center fixed;
}

div#content::before {
  content: "";
  position: fixed;
  top: 0; left: 0; right: 0; bottom: 0;
  pointer-events: none; /* lets clicks pass through */
  z-index: 0; /* behind your content */
}