Modal overlay created using CSS box-shadow.

index.html

<div class="modal">
  modal content
</div>

style.css

.modal {
  background: red;
  color: rgb(255, 255, 255);
  width: 25%;
  padding: 1em;
  position: relative;
  top: 50%;
  left: 50%;
  transform: translate(-50%, 50%);
 
  box-shadow: 0 0 0 9999px rgba(0, 0, 0, 0.5);
}