Has Bill Murray been visiting your vault? Is it by chance Groundhog Day?
Anyways, seriously... The only way the decor appears is to upgrade a level 2 three-room diner (unconfirmed?) or barracks (confirmed) to level 3. The decor doesn't just appear passively on existing rooms, it seems.
The costumes are also drops from lunchboxes pretty much. Or do you mean they all -physically- look the same? Or something else?
At 200 population it's impossible to delete and then upgrade a dwelling without killing and removing, in my case, 40 dwellers...