See Guide 8.12 for the anchors to be able to edit the Battle Terrain Backgrounds

How To

Terrain is set via var 0x5000

0 - Clear Terrain

1 - Electric Terrain

2 - Grassy Terrain

3 - Misty Terrain

4 - Psychic Terrain

NOTE: This var clears after battle

Example: setvar 0x5000 X with X being the terrain you want to set

Happy Hacking!