Rossmans at the main station has almost anything you need and at good prices. Open Sundays and holidays until 22:00. (they do have shorter hours on the 24th and 31st of Dec.
A small REWE on the Kaiserstrasse is open Sundays from 08:00-22:00, other days from 8-24:00. Their selection is pretty extensive for a small store.
The large Asian store, Gutleut Asiamarkt located at Kaiserstrasse 72, is open 12-16:00.
On the underground level of the Hauptbahnhof, across from Rossmans, is a smaller Asian store that is also open on Sundays.
Remember to wear your mask and keep a good distance from other shoppers.
Information about Sunday Shopping in all of the German Länder