#mask{
    position:absolute; /* important */
    top:0px; /* start from top */
    left:0px; /* start from left */
    height:100%; /* cover the whole page */
    width:100%;  /* cover the whole page */
    display:none; /* don't show it '*/

    /* styling bellow */
    background-color: black;
}

.modal_window{
    position:absolute; /* important so we can position it on center later */
    display:none; /* don't show it */

    /* styling bellow */
    color:white;
}