Edit the sprites to make your custom theme.
Be sure to keep them within the size limits.

You can also add custom music for both edit and play modes,
Though WARNING: Custom Theme Music is a little unstable right now.
Be sure that both of them have the exact same length,
or there will be problems with looping.

If no music is added, the default Ground theme will play.

If it's a water-based theme, create a TXT file named "water.txt"
in this folder, then you will be able to swim through the whole level.