Often used with Optifine (highly recommended for 1.12) to force light levels to stay at their maximum regardless of the environment.
The installation method depends on whether you prefer a resource pack or a dedicated mod. Method 1: The Resource Pack (Easiest) FullBright - Minecraft Resource Packs - CurseForge fullbright 1.12
Overrides Minecraft’s internal gamma setting beyond the normal max (1.0) up to 5.0 or 10.0, eliminating the need for Night Vision potions. Often used with Optifine (highly recommended for 1
There are three main ways to enable full brightness in Minecraft 1.12: How To Turn Up Your Minecraft Brightness To The Max fullbright 1.12