Local Gallery Item Pop-up Functionality

[ScriptAutomate]

When viewing the site https://tomanistor.com/ it looks like the site is using a modified version of the Osprey theme, because the gallery functionality I see with the GitHub theme allows for links that head elsewhere while the tomanistor site has a window pop-up with expanded information per gallery item, not merely links.

Is this an existing feature I'm not understanding the implementation of, or is this something that hasn't been introduced into the theme?