This is Dark Mode, not your typical "DARK MODE GUI", no, this makes EVERYTHING dark. Your whole world will be shrouded in darkness, and mobs will run rampant. That is, unless YOU do something about it!
Simply hold ALT while selecting the world generator, and go past the debug world!
All dimensions have their own unique changes! The basics are as follows:
Spoiler
Overworld is completely dark.
The Nether is now The Norther, and it has frozen over.
The End is now The Haven, and it is a (not very) hostile paradise.
Full credit goes to 30Spence for developing the original datapack.