My feet they are sore, and my limbs they are weary;
Long is the way, and the mountains are wild;
Soon will the twilight close moonless and dreary
Over the path of the poor orphan child. (Jane Eyre)
Ought the highlighted be called complement for subject, or complement for verb?
That's a pretty good question. It is a lot like "The gate swung shut", which was thoroughly discussed here.
But, to make it easy, I would say that the words "and it will be" are understood, in which case would be ellipsis and "moonless and dreary" would be , in R-K terms, a compound predicate adjective -- one of the SUBJECT complements.