God dispels the darkness of ignorance and hence we are sure that at least he is not darkness.
But then is He light?
We cannot apply the idea of our limited mind to God. God is beyond the imagination of our mind. Nor we can think of him as something black or white as in we cannot say that if He is not Darkness then He must be light.
So who is God?
vedanta Sutra says
JanmaDyasya yatah (1.1.2)
Meaning:From whom birth etc.
God is He from whom everything emanates i.e. from whom light, darkness etc everything has come.Everything that our mind can possibly think of and not think of emanates from Him only.
Thus it is not correct to apply the idea of just light to God. He is beyond it. Because He is the very source of both.
What is light?
In very simple terms light is energy of God; Material energy to be specific. And God is energetic. Just like Sun is the Source of Sunshine and Sun Shine is his energy.
In Gita Krishna Says
By Me, in My unmanifested form, this entire universe is pervaded. All beings are in Me, but I am not in them. And yet everything that is created does not rest in Me. Behold My mystic opulence! Although I am the maintainer of all living entities and although I am everywhere, I am not a part of this cosmic manifestation, for My Self is the very source of creation.
So Krishna says that by His energy he pervades the entire universe and yet He is not the part of this material cosmic creation. he is beyond it. He is fully spiritual.
So to answer your query in Nutshell.
God is neither light nor darkness but the very Source of both. Light or Darkness are just expansion of His energy or one of His unlimited energies.