"Right" is a funny choice of words, since if anyone has a right to choose the appearance it's the app developer. I'd have thought "option" more appropriate.
Anyway you'll have to ask Google. But my guess is "no" (actually my guess is that they won't even answer). You may be able to find a modified version if you search around though - there are people who produce dark themed versions of apps, for example.
Or just use a different email client that does let you customise the appearance.