.modal-dialog .overlay {
    display: -ms-flexbox;
    display: flex;
    position: absolute;
    left: 0;
    top: 0;
    bottom: 0;
    right: 0;
    margin: -1px;
    z-index: 1052;
    -ms-flex-pack: center;
    justify-content: center;
    -ms-flex-align: center;
    align-items: center;
    background-color: rgba(0,0,0,.7);
    color: #666f76;
    border-radius: .3rem;
}

/* remove the below if you don't want smooth scrolling */
html,
body {
    scroll-behavior: smooth;
}