It would be great to see K-9 Mail in pitch black colours and be optional to choose by going to Settings/General/Display/Theme/AMOLED (Pitch Black).
That’s been discussed in the forum (somewhere; can’t find the topic in the moment).
TL;DR: It makes basically no difference energy-wise, just a very few minutes over the day.
Or is it an optics/esthetics thing for you?
I’d rather say that it’s an aesthetic functionality for me.