Thank you so much for this guide, Lglaplante! Everything works nice for me.
I have experimented with the files a bit and I can give you all a list of which file corresponds to which background scene:
goo (dot) gl (slash) vy8nVB (It's a google document)
But that's not everything! I could also find out that "dec_155.bin" is the file which corresponds to the glowing rectangle which floats around the Pokémon in a WiFi Battle. So by replacing this file you can add other special effects (like spotlights via 157) or you can combine two scenes to have a background for your foreground to avoid glitches in the background.
Have fun with my list!
PS: I was not allowed to post a real link because I am new to GBAtemp. This is no spam but a helpful link, so please be kind to me, oh mighty Mods