In Android Studio, I can create any new project, go into Theme Editor, and see this problem. Here's a picture:
There are boxes around lots of the individual parts. They are colors like black and the accent color. The app renders normally when actually running on my phone, but the Theme Editor seems to be having a problem. It doesn't matter what the parent theme is. I'm running Android Studio 3.0. The two raised buttons aren't rounded and are both on one rectangle. Reproduction may not work on your system. I'm wondering what the problem is and how I can fix it.