The darkness represents the waters, which presented a great danger to those living on the islands, but also provides them with a large portion of their food.
The light represents the land, which gives the humans a place to live and farm, as well as hunt. It is just as important as the water, if not, more so.
I believe it is meant to represent the balance between light and dark as another way to show the way Hawaiians balance the land and water as a part of their lives. Important plants and animals come from both, and are used by the islanders for daily survival.
The story really discusses how everything comes from either light, dark, or parts of both. Nothing that comes from either is described as evil, or shown to be evil, but rather, something that is necessary for the islanders for consumption and survival. Everything that is created is created to be a part of their daily life. After everything necessary for their survival was created, the humans themselves were created, so that they may spread across the Earth and thrive. It shows that, like every other creation story we've seen in this class, that the humans believed they were created for some special reason, and that whatever that reason may be, it separated them from the animals and made them more important.