css overlay w3schools code example

Example: 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;
}