I know that a lot of non-native speakers tend to put everything inanimate in the 9/10 class, which would make it "njema" the form for inanimates with "mwema/wema" used for people and animals.
I'd expect something like "good night" to be common enough to avoid this though as common phrases often conserve grammar that is lost elsewhere in a language.
Probably time is the difference. Asubuhi njema would be said in the morning, and siku njema in the afternoon. But that's a guess. Mchana mwema is have a good afternoon, which would probably said in the afternoon. Is siku njema more or less generic for the day from late afternoon to early evening?
~ema changes more than other adjectives when in a different class: 1/2: mtu mwema, watu wema 3/4: (mti mwema, miti myema - however, all in parentheses do not make sense, but I put them for purpose of completeness) 5/6: tunda jema, matunda mema (another nound would be better, sorry, I couldn't find one off the top of my head) 7/8: kitu chema, vitu vyema 9/10: safari njema, safari njema (watch out, the animals's adjectives and verbs agree with class 1/2 as they are alive, the rest in concordance of animals is in this class) 11(/10): upendo wema 15 (verbs): kupenda kwema (place classes:) 16: (pale/mahali pema) 17: (kule kwema) 18: (hamo mwema) 17 and 18 have no nouns in them.