For a Fullbright texture pack to function correctly in version 1.12.2, you usually need to configure your video settings or use a performance mod. The OptiFine Requirement
To install a resource pack, go to Options > Resource Packs > Open Resource Pack Folder and drag the downloaded .zip file into that folder. Then, move it to the "Selected" column in the game menu. FullBright - Minecraft Resource Packs
For a Fullbright texture pack to function correctly in version 1.12.2, you usually need to configure your video settings or use a performance mod. The OptiFine Requirement
To install a resource pack, go to Options > Resource Packs > Open Resource Pack Folder and drag the downloaded .zip file into that folder. Then, move it to the "Selected" column in the game menu. FullBright - Minecraft Resource Packs fullbright texture pack 1.12.2