Do you think that some vanilla textures are ugly and do you want to change them? Well, this tutorial is exactly for you.
Background explanation
The src folder in the workspace is the place where texture and models go, similar to texture packs. The generator will create blocks, entities, particles, and items, and the mod's "texture pack" texturizes them. But if you change the directory of files by adding some folders, this will become a texture pack at all!
Changing vanilla textures
First, you need a texture for what you want to change (entity, block, etc.) and go to the Workspace folder. Then go to "src/main/resources/assets" and create a new folder and call it Minecraft. Into the Minecraft, folder create two folders and call them textures and models. Into textures and models, folders add another folder and call it "item", "block", etc. based on what do you want to change. Then put the folder your texture and call it with the exact name of the precedent texture. For example, if you want to change sand texture your texture file name has to be sand.png. Now restart Minecraft and see the result.
Notes
- Without Optifine you can't change entity models
- Don't put older Minecraft version textures without being sure that works
- If you want to tweak something else remember that is like a texture pack so read here for more info https://minecraft.wiki/w/Resource_pack