It seems that the FileMaker 16 runtime engine (on the Mac, at least) still preserves the Classic theme in a way it shouldn't.
1. Create a FileMaker file from scratch in FileMaker Pro Advanced 18.104.22.1682 on macOS 10.12.16.
2. Create a runtime for that file.
3. Open the runtime file and export data using File > Export Records..., using the "FileMaker Pro" Type.
4. Open the file exported with FileMaker Pro Advancec 22.214.171.1242: the layout in the file uses the "Classic" theme