Should the kitchen floor be lighter or darker than the countertop?

Neither is a rule. The reliable approach is contrast without competition. If the countertop is light, a mid or deeper floor grounds the room. If the countertop is dark or heavily veined, a lighter floor keeps the space open. What causes trouble is a floor and countertop close in tone but different in undertone, one warm and one cool, because the eye reads that as a mistake rather than a match. Put samples of both together, in the actual room, in daylight, before committing.