How to add custom tilesets

I've found the tileset folder, but adding a new tileset does nothing in the editor.

You cannot ADD a tileset because the in-game editor only goes from 0 to 255. You can REPLACE a tileset though.

If you create a tileset folder in your level, you can then place tileset000.png (or replace the 000 with any number between 000 and 255) to replace the tileset in-game.

Do note, you do not want to replace the tileset in the world folder, because that will affect every level, and makes it so that if you ship your level to others, they won't see your tileset.