The answer could be one of two words. If they treated each other with respect, then the choices are
civility: meaning formal polite respect.
decorum: treatment with good taste
I don't think there's another choice and of those two, I don't know which one to pick since they both mean pretty much the same thing. In addition, both are nouns, and both refer to behavior.
This is an example of a simile, a comparison between two things using the words "like" or "as." "Gloomy as the night" is the simile here, since his sadness is being compared to nighttime.
