defaults write /Applications/FileMaker\ Pro.app/Contents/Info NSRequiresAquaSystemAppearance -boolean yes
If you have FileMaker Pro installed in a different location, you may need to adjust the path. To reverse you can always call this again with "no" as value on the end.
If a future FileMaker 19 update replaces the application, this setting is reset.
PS: This may break code signing, so you may also use codesign --remove-signature command to remove the broken signature.