Find the line gamma:1.0 (or similar) and change it to 100.0 .
Find a compatible file on community sites. One highly-rated option is the Fullbright 1.12 by World Resource Pack on Planet Minecraft. Place the File: Open Minecraft 1.12.2. Go to Options > Resource Packs > Open Resource Pack Folder . Drag and drop the downloaded .zip file into this folder. minecraft 1.12.2 fullbright texture pack
A highly popular choice on CurseForge that specifically supports version 1.12.2. Find the line gamma:1