how to make an overlay css code example

Example 1: make background overlay css

#element-with-background-image {
   position: relative;
   background-image: url("//spin.atomicobject.com/wp-content/uploads/20170324102432/portfolio-tips-feature-image.jpg");
}
 
#color-overlay {
   position: absolute;
   top: 0;
   left: 0;
   width: 100%;
   height: 100%;
   background-color: black;
   opacity: 0.6;
}

Example 2: 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 3: 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*/
}

Example 4: overlay a box in css

html, body {
  min-height: 100%;
}

body {
  position: relative; /* needed if you position the pseudo-element absolutely */
}

body:after {
  content: "";
  display: block;
  position: fixed; /* could also be absolute */ 
  top: 0;
  left: 0;
  height: 100%;
  width: 100%;
  z-index: 10;
  background-color: rgba(0,0,0,0.2);
}