I always preferred the nighttime setting of Wily Castle in the Wii U version of the game, and, for the life of me, couldn't see why they changed it to daytime for the 3DS Version. Unhappy with this, I made a mod that (partially) rectifies that.
Only the Omega stage has been changed to take place at night. I tried editing the normal version of the stage, but unfortunately, I had a few issues getting the textures to work, so I ended up leaving it as is. The Stage Select UI has now been slightly updated to acknowledge the fact that the Omega version now takes place at night.
It's a bit basic, but not bad for a first Smash 4 mod imo.