This sentence is awkward in English.
Yeah....it actually says that the restaurant became evening - as in, the restaurant turned into evening.
I really hoped I would have 'got it' by this late stage in the course, but apparently not. Why is it det, not den? Restaurant is common, as is aften, so why the neuter-form 'it'?
It is a formal subject, like in the English "It is raining.". Det it used as the formal subject.