"Isn't it dark in the evening?"
I would say that よる is later than ゆうがた, since よる can refer to either night or evening, while ゆうがた means just evening.
Kurai or kurakunai? Wouldn't kurakunai means opposite of dark; i.e., bright?
'Isn't it dark' is generally asking for confirmation on the fact that it is dark, not that it isn't dark. It's like saying "isn't he handsom".
Well, I looked it up and I found “daytime” as an answer and if I recall correctly, Duolingo said it was “afternoon” some lessons ago. 昼間