STEP 1: Load into a level like a raid.
STEP 2: Physically walk over to your keyboard.
STEP 3: Press left or right arrow to decrease or increase level brightness, like raising the sun essentially.

There's no extra options or hassle to enable it or anything and the game doesn't really tell you it's an option. You just press left or right arrow.
Congrats. Now it's not pitch black darkness. Have fun! Found this out cause I wanted to do some recordings.