Expanded gallery images hidden by top menu!

I’m having an annoying problem with my top menu conflicting with my gallery images. When I open a gallery image to view it, the top part is tucked behind the top menu, hiding the X button, making it impossible to close the image and cutting off part of the image. I need the open gallery image to be ON TOP of the top menu, but I don’t know how to change the code to do so. I’ve tried making the menu fixed (not sticky on scroll) but that doesn’t make any difference.

You can see what I mean by going to this page and clicking on one of the thumbnail images in the gallery: https://valemounttrails.com/big-foot-trail/

I’m doing this website for another organization so I need to fix this issue. Any help would be appreciated. Thank you!