Battle Background Customisation
Since Pokémon X & Y introduced the notion of a 3D stadium in which to fight in multiplayer battles, players have been locked in to the specific battle background for multiplayer battles locally and online. However, Pokémon Sword & Shield change this up by allowing you to have multiple different battle backgrounds.
This can be changed at will but only by your League Card or Profile ICon. If you have a League Card with a specific background or the profile icon matching the gym, it will give you a 50% chance of having the battle in the Stadium that corresponds with your League Card/Icon. If both players have the same background/Icon, or a background/Icon of the same category, then it's guaranteed that your battle will have the specific stadium. The lighting will also vary depending on the time of day in the game.