Updated instructions for the texture "pack":
How to install:
1. Copy the "AntiqueAtlasExtra.zip" to the mods folder
2. Replace the files "biome_textures.json" and "texture_sets.json" in the folder "config/antiqueatlas"
This is not exactly a texture pack in the sense that vanilla texture packs are, so it won't show up in the list, but it will be in effect immediately.