All of the theaters at DHS have been using the same costume for many many years - Fantasmic, Theater of the Stars, Lightning McQueen's Racing Academy, Frozen, Indiana Jones, Jedi Training, Path of the Jedi, etc. It was a blue and grey camp-style shirt and black pants. The new costume technically fits the park more - it is quite literally a color palette that is inspired by the main entrance/security/skyliner/bus shelters.
Mermaid and Disney Jr. have now adopted the same look. They had been using a red shirt and khaki shorts/pants combo. This costume had been used as the "backlot" parts of the park - Backlot Tour used this same generic look before it closed.
My point is, all they did was change "what" the costume is. It's not getting rid of some sort of thematic costume and replacing it for another. None of these areas had their own, individual themed costumes before hand. The theaters have always had the same costume. This isn't taking away the bellhop costume and replacing it with a polo that says "Disney's Hollywood Studios" on it.
The only location of this new costume that may be a bit controversial is Muppetvision. But, again, the new idea is that the theater is Grand Arts Theater, presenting MuppetVision 3D. The show is no longer being held "at" Muppet Labs. Now, whether or not this makes sense with the story presented in the movie is a COMPLETELY different topic. But since a themeing decision has been made that, at least from teh outside, you are not supposed to be at the Muppet Labs, but are instead at the Grand Arts Theater, it makes sense that the costume is the "theaters" costume. It's the same way that it's Theater of the Stars presenting Beauty and the Beast Live on Stage, or the Harambe Theater presenting Festival of the Lion King, or Theater in the Wild presenting Finding Nemo the Musical. The costumes at those venues aren't inspired by "what" is being presented inside, they are inspired by the theater's story-telling.
Even the area-specific costumes are not a new thing. Tomorrowland attractions (minus the speedway) all use the same costume, and have for many many years. Many areas of Studios food & beverage (particularly ODV) have used the same costume - despite where they are located.