This was a tweak that was mentioned for the demo version of the game to tweak the overly bright lighting and gamma, so this config edit can be used on the game to adjust the gamma for the game. I'm using it on the full game and it works too and gives the game a bit more dark atmosphere if adjusted right for your display (1.6 display gamma seems like a good balance with brightness of 6).

• Source: https://www.reddit.com/r/Bloodstained/comments/8vcdbd/darker_bloodstained_ritual_of_the_night/