A lot of people confuse theming and decor.
Theming: decorating a room (or an area in a park) in a way that takes you into a different place/time (the theme) and helps you pretend to be there.
Decor: putting up nice pictures of nice things and characters.
The Star Wars Hotel at WDW was the epitome of a themed hotel. The new ships are mainly just decorated. Having a picture of Tiana above your bed does not make the room Tiana themed. Tiana can’t even be a theme, certain places in her movie can be though. So you could have a bayou themed room with shelves looking like they were made out of branches, a carpet that looks like grass, fireflies in the ceiling etc.
Theme also can be totally independent of any characters or movies, think the World Showcase pavilions in Epcot - or the four original ships that are themed to the theme classic ocean liner.