#lightbox {
  position: fixed;
  left: 0; top: 0; right: 0; bottom: 0;
  background-color: rgba(0,0,0,0.9);
  display: none;
  z-index: 9999;
}
#lightbox.visible {
  display: block;
}
#lightbox > * {
  position: absolute;;
  left: 10%; top: 10%; right: 10%; bottom: 10%;
  background-position: center center;
  background-repeat: no-repeat;
  background-size: contain;
}