Any option to change the subtitles appearance?

I only see the default subtitles as white text on a semi-transparent background. That is fairly annoying imho and I wonder if there is a secret setting that would allow to make the background completely trasnparent and maybe add black outline text (or shadowed). If that's not possible within the app, would it be possible to have UHF honor the Apple TV's subtitle settings? Those are highly customizable.