I've been trying to figure which item will change the park into the Anniversary theme to pass on traits from Makiko and Nemunemuutchi. I noticed whenever you m!x a tamagotchi with a tama town character, that trait is always more dominate, so the anniversary background is only during the earlier stages.
I can't get the photo to attach, so I included a link. For reference, the background has hearts on the grass patches, and glowing bow lamp posts. If the item required is a M!x station item, let me know because I've tried almost every item in the shop.