I found both dark and light rye bread at the store but it was the factory-made kind. I definitely made mistakes with both batches including adding way too much bread to the light kvass.the bottles need a day to carbonate then we shall see how well they turned out. Not that I have any reference for the flavor.
Kvass is a Slavic drink highly popular in the Slavic countries. It is lighter than beer (considered non-alcoholic in most places, alcohol content is typically below 1,5%vol), very refreshing and just nice. You should give it a try!