"Is it dark in the evening?"
You don't have to thank Duolingo for that. You have to thabk English and Japanese for that.
As far as I know 夜 means "night" as in "the time when it's dark outside". 夕方, according to tangoa on italki means "In daily conversation, it is used to mean the period around the sunset time, from when it starts to become dark until it is completely dark. So it varies from season to season."
In English people hardly ever use the word "dusk" in a normal conversation. It's more common to use "evening" or "night". There is some overlap in these words and the time in which you can use them. So if the contributors on Duolingo make a sentence with 夕方 then the only proper English translation would be "evening". But if you translate this back into Japanese you could use the wrong translation and change the meaning of the sentence. So I think it's more that language is just messy and inconsistent instead of Duolingo being inconsistent. (But of course you don't have to agree :) )