.center-box-overlay {
    display: table;
    position: fixed;
    top: 0px;
    left: 0px;
    height: 100%;
    width: 100%;
    margin: 0px;
    
}

.center-box-window {
    display: table-cell;
    vertical-align: middle;
}

.center-box-content {
    margin-left: auto;
    margin-right: auto; 
    width: 600px;
}