<div class="overlay">
	<div class="lightbox"></div>
</div>

.overlay {
  position: fixed;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  background-color: rgba(0, 0, 0, 0.6);
}
.lightbox {
  z-index: 1;
  /*垂直与水平居中*/
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}