"Fine, good night."
Just as "dobry", "fajne" is also an adjective. Eventually You could use "fajnie", though remember that in Polish this word and its inflections are colloquial.
Yeah, but firstly - it's colloquial, and secondly - it really is too much for 'fine'.
Does dobranoc also mean good evening or is there a different phrase/word for that?
"good evening" = "dobry wieczór".
"dobranoc" is definitely wishing someone a good night as a goodbye.
That is totally colloquial. OK, "fine" isn't formal, but "spoko" is something you can only say among friends...
"dobranoc" could potentially be "dobra noc", but the phrase was just formed that way over the centuries.
"do widzenia" has 'do' separately because it means 'until'. It's like "until we see each other (again)".