Gallery Thumbnails Showing Up Above Lightbox Image

So, I’m using the NextGEN Gallery plug-in. However, when I click on a thumbnail to bring up a lightbox version of the image, the thumbnails show up on top of it. I’ve looked all through the CSS, including z-index values and all, and I can’t figure out what’s going on. Here’s the site:

https://www.kindredbody.com/gallery/

Thanks.