css make something overlay everything code example

Example 1: css overlay

#hero{
  background-image:url();
  background-size:cover;
  background-position:top center;
  position:relative;
}

#hero::after{
  content:'';
  position:absolute;
  left:0;
  top:0;
  height:100%;
  width:100%;
  background-color:black;
  opacity:0.7;
}

Example 2: overlay a box in css

html, body{
  min-height: 100%;
}
body{
  position: relative;
}
.overlay{
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  z-index: 10;
  background-color: rgba(0,0,0,0.5); /*dim the background*/
}