.modal-backdrop {
   background-color: grey;
    z-index: 0 !important;
}
body {
  position: fixed;
}
.modal-dialog {
    -webkit-transform: translate(0,-30%);
    -o-transform: translate(0,-30%);
    transform: translate(0,-30%);
    top: 30%;
    margin: 0 auto;
}