Website always has awful image viewer that won't go away

My company’s website has a custom theme that was designed for us, and it is a child of OVERall theme. No matter which plugins we install and activate, we always have the same awful fullscreen image viewer. I was told I am not allowed to put a link to the site, but the basic interface of the viewer is this:

When a thumbnail is clicked, the screen darkens and the photo appears centered with captions below it. The only way to advance to the next picture is to click small left/right arrows on the bottom of the page, where there is also a red circle between them with a white X to quit, and a folded arrow to show the image in its default resolution if it has been shrunk. One can also close the fullscreen viewer by clicking on the image itself, but clicking on the darkened portion of the screen does nothing.

The interface is very non-intuitive, and it does not look very professional.

Is there a way to override this?

Has anyone come across this?

We have tried Gamma Gallery, NEXTgen gallery, Fullscreen Galleria, and several others. Some of them altered the thumbnail arrangement, but our fullscreen viewer is still the awful, nasty one.