overlay in css

  #overlay {
  position: fixed; /* Sit on top of the 
  page content */
  
  display: none; /* Hidden by default */
  width: 100%; /* 
  Full width (cover the whole page) */
  
  height: 100%; /* Full height (cover the whole page) */
  top: 0; 
  left: 0;
  
  right: 0;
  bottom: 0;
  
  background-color: rgba(0,0,0,0.5); /* Black background with opacity */
  z-index: 2; 
  /* Specify a stack order in case you're using a different order for other 
  elements */
  
  cursor: pointer; /* Add a pointer on hover */
}