How to vertically align Bootstrap v4 modal dialogs

Question

Vertically center modal dialogues in Bootstrap 4.

Note: The requirements below have been added to make it clear I am looking for a prop

Answer 1

By adding following css to '.modal-dialog' class, it works fine for me. You can override with custom css class also.

.modal-dialog {
      height: 100vh;
      display: flex;
      align-items: center;
}