button.close {
    position: relative;
    z-index: 1;
    right: 0.5rem;
    top: 0.5rem;
}

.modal-dialog-centered {
    display: -ms-flexbox;
    display: flex;
    -ms-flex-align: center;
    align-items: center;
    min-height: calc(100% - (0.5rem * 2));
}

.modal-dialog-centered .modal-content {
    width:100%;
}

@media (min-width: 576px) {
    .modal-dialog-centered {
        min-height: calc(100% - (1.75rem * 2));
    }
}