Here is everything you need to know about why this "utility" pack is essential and how to get it running. What is a Fullbright Texture Pack?
While Minecraft has moved on to much newer versions, 1.12.2 remains the "Golden Age" for modding. Because many players use 1.12.2 for technical gameplay or competitive PvP, having a Fullbright pack is vital for:
In standard Minecraft, light levels dictate what you see. Without a light source, caves and nighttime environments are pitch black. A Fullbright pack modifies the game’s internal brightness parameters, effectively turning the "gamma" up to the maximum.