mobile: center second row of gallery grid?

Try this CSS code to Website > Website Tools > Custom CSS

@media screen and (max-width:767px) {
div#block-f4af4926eecb45ea1960 .sqs-gallery {
    display: flex;
    justify-content: center;
    flex-wrap: wrap;
}
div#block-f4af4926eecb45ea1960 .slide {
    width: 33.3333% !important;
}
}