Https://www.dessertshouse.com/

https://www.dessertshouse.com/
need to add an outer line for product thumbnail how can do it regards

Sorry, I don’t understand what you mean. Can you explain it more clearly with pictures?

You can fix it with the CSS:

.singular-shop div.product .flex-control-thumbs .slick-list li img {
    border: 2px solid red;
}