The Greek words ho theos refer to THE FATHER. Jesus sat down on the throne of his Father. The word throne at Hebrews 1:8 is not a reference to a chair but to the Kingly authority of God (see Luke 1:32). When Jesus sat down on the throne of his God and Father, his authority was the authority of God, the right to execute his Gods authority. And that is why the text says Your throne is God. Yes it is indeed. These words will make a Trinitarian brain cease right up due to their misguided preconceptions. But this is Biblical language. The Davidic King sat on the throne of God which means he executed Gods authority over the Kingdom of God (1 Chron 29:23; 28:6). And that is why Psalm 45:6-7 refers to an ancient Davidic King and can also be applied to Jesus, a son of David. That ancient Davidic King is not God and neither is Jesus. The Davidic King, including Jesus the son of David, sits on the throne of God; he executes Gods authority. This is what Hebrews 1:8 is talking about. The risen Jesus sat down at the right hand of God which means he was given the authority of his Gods throne.