Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a 9x13-inch baking dish.

Mix together Thousand island dressing, sour cream, and onion in a medium bowl until well combined.

Arrange dark and light rye bread cubes in the bottom of the prepared baking dish, reserving about 1/2 cup each for topping. Top bread cubes with corned beef and sauerkraut. Spread dressing mixture evenly over the top. Sprinkle with Swiss cheese, top with reserved bread cubes, and drizzle with melted margarine.

Bake, covered, in the preheated oven for 15 minutes. Uncover and continue baking until bubbly and lightly browned, about 10 minutes more.