It is not possible to customize the appearance of the icons. To move the icons to the bottom, you can add this CSS rules to the styles.scss file:

.carousel-control-prev-icon {
    position: absolute;
    bottom: 20px;
    left: 20px;
}

.carousel-control-next-icon {
    position: absolute;
    bottom: 20px;
    right: 20px;
}