The closest possibility to buy food is at the petrol station AO, which is around 12minutes drive away. They only have a very limited basic assortment (more expensive as in supermarkets) and are open until 8PM daily.
If you want to buy proper groceries, we recommend to drive to Mosfellsbær or Akranes at Bónus (cheapest supermarket) or Krónan (more variety, normal price). If you want to get alcohol there’s only one store chain in Iceland. It’s called vínbúðin and is closed on Sundays and holidays and also closes at 18:00 every other day.