Thanks everyone - Stockholm was great, go midsummer sun! Went with Taxikurir for the way in and indeed the rate was fixed and same as the daytime rate (SEK 490 around 12:30am). Even though the walk from Central Station to the hotel was only 5-10 minutes walk, at that time of night and first time in city, the taxi was a good deal.

Way back took the Arlanda Express 2for1 at SEK 280.