If you are looking for an FMX style that says "professional", take Blend out for a spin. The Blend style looks great in FMXLinux, and it is included in Delphi 10.3 Rio.
Simply open a TStyleBook to use the Linux version of Blend, found in the Styles folder:
Here's some examples of Blend in action:
Blend is also available for use in your Windows, iOS, Android, and macOS projects, all included in Delphi 10.3 Rio