#modal_wrapper.overlay:before {
  content: " ";
  width: 100%;
  height: 100%;
  position: fixed;
  z-index: 100;
  top: 0;
  left: 0;
  background: #000;
  background: rgba(0,0,0,0.7);
}
#modal_window {
  display: none;
  z-index: 200;
  position: fixed;
  left: 50%;
  top: 50%;
  width: 50%;
  overflow: auto;
  padding: 10px 20px;
  background: #fff;
  border: 5px solid #999;
  border-radius: 10px;
  box-shadow: 0 0 10px rgba(0,0,0,0.5);
}
#modal_wrapper.overlay #modal_window {
  display: block;
}