page overlay with css code example

Example 1: div color overlay css

box-shadow: inset 0 0 0 2000px rgb(0 0 0 / 67%);

Example 2: 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);
}