This is about the majesty of God. Here is the powerful throne, the throne of the everlasting God of love, and yet he is a just God, and he will bring to judgement all sin and all rebellion.
The Spirit of God is that perfect Spirit who gives the true light of God to men, of which the light of the sun is only a faint reflection. Why is truth like a light or a torch? Because it reveals what is real and what cannot be seen without it. Nothing that is inconsistent with truth can survive. Truth is what men and women should hunger for. The Spirit of God is the source of all truth to us, being the member of the Godhead who is responsible for communicating truth directly to us. He does not derive that light from any other but he has it within himself. Truth and the revelation of truth flow from within him. The throne of God is bright because of the light that surrounds it.