I'm trying to change a few of the maps for my hack, and have realized that the battle background is determined by map number, not tileset.
That seems odd, because in dungeons the background changes based on the tile you're on.
00: Battle/Spell Properties bits 0-3: Battle Background 0: Grass 8: Magnes/Sealed Cave 1: Forest 9: Desert 2: Mountain A: Beach 3: Castle B: Land of Monsters/Sylphs 4: Cave C: Tower of Zot 5: Moon D: Lunar Subterrane 6: Ship E: Crystal Room 7: Water F: Underground bit 4: Warp can be cast bit 5: Exit can be cast bit 6: Use alternate palette if available (backgrounds 0, 4, 7, 8, B, C) bit 7: Battles have magnetic effect if event flag E1 is set