style.css

#modal {
  z-index: 1;
  display: none;
  position: fixed;
  left: 0;
  top: 0;
  width: 100vw;
  height: 100vh;
  background-color: rgba(0, 0, 0, 0.9);
  cursor: pointer;
}

/* Modal border */
#modal_inner {
  position: absolute;
  top: 40px;
  bottom: 40px;
  left: 40px;
  right: 40px;
}

/* No modal border on smaller screens */
@media only screen and (max-width: 700px){
  #modal_inner {
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
  }
}

#quantized_img {
  z-index: 2;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%,-50%);
  max-width: 100%;
  max-height: 100%;
  cursor: auto;
}

#close {
  z-index: 3;
  font-family: Arial, Helvetica, sans-serif;
  position: absolute;
  top: 15px;
  left: 35px;
  color: #f1f1f1;
  font-size: 60px;
  text-shadow:  1px  1px 0 rgba(0, 0, 0, 0.9),
                1px -1px 0 rgba(0, 0, 0, 0.9),
               -1px  1px 0 rgba(0, 0, 0, 0.9),
               -1px -1px 0 rgba(0, 0, 0, 0.9);
}

#close:hover, #close:focus {
  color: #bbb;
  text-decoration: none;
  cursor: pointer;
}