Place white bread flour, whole wheat bread flour, and yeast into a large mixing bowl and stir with a wooden spoon. Pour in cold water and mix with a wooden spoon or spatula until a very wet, sticky dough comes together, about 3 minutes. Add the salt and mix another 2 to 3 minutes. Scrape down the sides of the bowl. Cover with foil. Allow dough to rise at room temperature for 18 hours.