How to give border to to current displaying border

I am using Shopify envy theme. I have multiple product images. I want to give border to thumb image which image is displaying as main image. In short the current image should have border.

Here is the example image what I want :enter image description here

Small images are multiple different thumb image. If thumb image is displaying in main big image that thumb should have border.

This is my jQuery:

$('a.image-swap').click(function() {
    var newImage = $(this).attr('href');
    $('.featured-image-div img').attr({ src: newImage });
    return false;
});


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