hack
[hak]
–verb (used with object)
...
7. Computers . to devise or modify (a computer program), usually skillfully.
Technically, modifying the game for any purpose is a hack. The problem is when people modify it (or download programs that modify it) for cheating. Modifying (or "hacking", if you want to call it that) textures in a way that makes players easier to see is a form of cheating.
You've admitted to modifying the sky textures. That's what you were banned for. Your ban will be reduced to 32 days.
