CODE EXAMPLE FOR CSS

full screen box shadow css

.modal {
  /* general styles */
  width: 500px;
  height: 200px;
  background-color: white;
  display: grid;
  place-items: center;
  
  /* overlay */
  box-shadow: 0 0 0 500vmax rgb(0 0 0 / 0.5);
}

/* a codepen to demonstrate: https://codepen.io/arq091/pen/RwMKOyZ */