create overlay using 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;
}
.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*/
}