.popup {
  display: none;
  position: fixed;
  padding: 10px;
  width: 400px;
  left: 50%;
  height: auto;
  top: 50%;
  background: #FFF;
  z-index: 20;
  text-align: center;
  margin-left: -200px;
}


#popup:after {
  position: fixed;
  content: "";
  top: 0;
  left: 0;
  bottom: 0;
  right: 0;
  background: rgba(0, 0, 0, 0.5);
  z-index: -2;
}

#popup:before {
  position: absolute;
  content: "";
  top: 0;
  left: 0;
  bottom: 0;
  right: 0;
  background: #FFF;
  z-index: -1;
}