FMX Styles are designed to give your product a clean, polished look. The Dark style is a terrific example of this, especially when it is used in FMXLinux, new in Delphi 10.3 Rio.
To use Dark, all you need to do is open a TStyleBook to include the file, found in your Styles folder.
I've included some examples of the Dark style on Linux here:
Dark is also a great option for your Windows, iOS, Android, and macOS projects.
Is there a converter to turn FMX styles into VCL styles?