Modding:Theme Pack
Theme Packs are mods that do not change the game mechanics, only visuals, sounds, etc. They are packed like any other mod, using the type "theme"
.
Due to the way they work, theme packs never add new content, only replace existing content.
Overriding Files
Replacing game files is fairly easy, just create a directory names "assets" with a directory named "game" inside it, then copy replacements for any asset in the game's "assets" directory here. For example, to overwrite the texture for low fertility soil blocks ("assets/textures/block/soil/fertlow.png") you would place your replacement at "assets/game/textures/block/soil/fertlow.png" inside your zip file.
Overriding assets from other mods is also possible, see the asset system page for more details. Check out the example theme pack for a simple demo of overriding game assets.