.modal {
        text-align: center;
}
 
@media screen and (min-width: 768px) { 
        .modal:before {
                display: inline-block;
                vertical-align: middle;
                content: " ";
                height: 100%;
        }
}
 
.modal-dialog {
        display: inline-block;
        text-align: left;
        vertical-align: middle;
}