/* Z-index of #mask must lower than #boxes .window */
#mask {
  display: none;
  position: absolute;
  top: 0px;
  z-index: 9000;
  background-color: #000;
}
  
#boxes .window {
  display: none;
  position: absolute;
  width: 400px;
  height: 240px;
  padding: 20px;
  background: #000;
  z-index: 9999;
}

/* Customize your modal window here, you can add background image too */
#boxes #dialog {
  width: 375px; 
  height: 203px;
}