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.
From what I understand you can then use the filtered out bread as a starter for the next batch. There is also beet kvass that is either combined with rye bread or used on its own. So far it seems like the easiest and most versatile probiotic drink style but I haven't experimented with all of them yet. I didn't check ABV but they say it is usually 0.5-2.0%