On 1.12.2, you can edit the options.txt file in your Minecraft directory. Change gamma:1.0 to gamma:1000.0 . This effectively simulates Fullbright. This is the safest and most direct method.
For version 1.12.2, Fullbright is often implemented as a resource pack that requires fullbright texture pack 1122 link
Fullbright texture pack for Minecraft 1.12.2 is a popular utility that allows players to see perfectly in the dark, including deep caves and at night, without the need for torches including deep caves and at night
) who want to navigate dark environments with maximum clarity. The Story of the Eternal Lantern In the deep, jagged ravines of a world stuck in version fullbright texture pack 1122 link