Night mode is a known issue: 
https://github.com/koreader/koreader/issues/3529
	Quote:
	
	
		| EDIT2: also, for some reason, downloading the "stable" release from github says it is corrupt. Won't install. | 
	
 It wouldn't work on Android 7 anyway. 

 Although that is curious.
Note that the calibre thing requires you're on the same network.