Is it just me or are all the images used in the screenshots extremely overbright, as if some HDR mode or a nuke was constantly set off?
Is it like this in the game, because I would have hoped that the actual cave is dark like a horror game. Any way the screenshots can be updated to reflect how the mod actually looks?

You can adjust the lighting in the mod settings. There are several options available, including Pitch Black, Always Night, Always Day, and Normal, so you can choose whichever atmosphere you prefer.

I used brighter lighting for the screenshots intentionally so people could clearly see the environment and all the details. If I had taken them with the darker settings enabled, most of the screenshots would have been too dark to properly show what is in the mod.

So yes, you can absolutely make the cave very dark and give it more of a horror-game atmosphere through the mod settings.