Some packs don't remove shadows; they invert the logic. Dramatic Skies keeps clouds and skyboxes but forces the "sun" light level on all block faces. This is great for recording timelapses of builds that are underground.
Launch the and boot up your 1.12.2 installation. minecraft 1.12.2 fullbright texture pack
Click and paste your downloaded .zip file there. Some packs don't remove shadows; they invert the logic