.woocommerce-product-gallery .flex-control-thumbs {
    list-style-type: none;
}

.woocommerce-product-gallery .flex-control-thumbs li {
    display: inline-block;
    width: 20%;
    margin: 0 2% 2% 0;
    background: #eee;
    padding-bottom: 20%;
    position: relative;
}

.woocommerce-product-gallery .flex-control-thumbs img {
    object-fit: cover;
    width: 100%;
    height: 100%;
    position: absolute;
}

@media (min-width: 600px) {
    .woocommerce-product-gallery .flex-control-thumbs li {
        width: 15%;
        margin: 0 2% 2% 0;
        padding-bottom: 15%;
    }
}