#overlay {
    display: none;
    background-color: black;
    position: fixed;
    left:0;
    top:0;
    height:100%;
    width:100%;
    opacity: 0.5; /* also -moz-opacity, etc. */
    z-index:1000;
}

#overlay_center {
    opacity: 1;
    position: fixed;
    left:50%;
    top:50%;
    /* width: 100px; */
    /* height: 100px; */
    /* margin: -50px 0 0 -50px; */
    z-index:1000;
}