How do I use javascript to dynamically add the fancybox class to gallery links?

I’m using an e-commerce theme that uses lightbox for product images. However, when I add a gallery to a standard “page” and click a thumbnail, the page reloads and shows just the photo. Instead, I want the thumbnails to open in lightbox or something similar. the theme creator said could use javascript to dynamically add the fancybox class to your gallery links and then call the fancybox jquery plugin to take over. How do I do this? TIA