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There were some unintended side effects with the brightness slider so it's disabled in the 1.31 build.  As for the tutorial, it's okay if people are looking for a while, it's very empty, eventually you'll spot the target.

As for the cave, I agree that it's dark. That's why we added the torch you can equip by pressing T. (see lower right bottom/tutorial message) 
Caves are still somewhat affected by the time of day outside which is unintended but the caves in game will be dark. That's why we added the torch. I hope to fix the brightness slider for the next build.

Luckily most people are in fact guided by the big god rays and lights placed around in the cave which do lead to the exit, like you said, players follow the lights ;)