This is the classic "Gamma 1000" exploit.
Select the pack from the "Available" list in-game and move it to the "Selected" column. Alternative: The "Gamma" Trick fullbright 1.12.2 resource pack
If you'd like, I can help you , explain how to pair it with shaders , or show you how to tweak the settings if your game still looks dark. This is the classic "Gamma 1000" exploit