Something very similar happened with my 16 year old daughter recently. Her dad is living in a condo, and his evil sister (her aunt) came to visit and do some decorating for him. The aunt redecorated my daughter's bedroom and didn't really incorporate my daughter's wishes. It was not a big deal overall ---- but it triggered a lot of confusing feelings for my daughter and it was not handled well. My daughter kind of blew up and everybody was upset. I think it triggered a lot of sadness over her parents splitting up, her dad being a drunk, and all that.
This teen girl has been through a lot. And she might be a total brat, but keep that in mind. Perhaps you could talk with her and ask her what is going on. Something like "Hey, you had some real strong feelings about that bathroom wallpaper. It's important to me that you feel like this is your home too. Can you help me understand what you are feeling?"
She may say nothing, she may tell you to f- off, but she will at least know that somebody cared enough to help her sort out some of her feelings.
It's just wallpaper. Perhaps she needs to have a say in this new house. It doesn't mean you have to cave to her demands or let her run the show, but somehow involve her a bit. Maybe you can involve her in figuring out how the bathroom should look. Be sensitive to her, even if she's a pain in the neck as most teens are. I have FOUR teenagers, and behind the placid surface of all of them is a lot of grief and hurt.