Colorbox Slideshow not working

I am experiencing the same issue:
https://torontochamberchoir.ca/relaunch/about-us/

I attempted to fix it by installing a plugin called Lightbox Plus ColorBox, thinking having that library somewhere in the plugins would allow the functionality to work. It did not. I have deactivated the plugin, and it still doesn’t work.

The theme I’m using is a custom theme built from a plain template. I do not think it has any built-in colorbox-style image handling.