#mask {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    height: 100%;
    width: 100%;
    display: none;
    background-color: black;  
}  
  
.modal_window {  
    position: fixed;
    display: none;
    color: #000;
    background: #fff;
    background-color: rgba(255,255,255,1.0);
    width: 800px; height: 100%;
    border: 1px solid black;  
    padding: 50px;  
    z-index: 1000;  /* make sure the modal window is right on top */
}

.modal_window h4 {
    text-align: center;
    padding-top: 10px;
}