This may seem simplistic, but here we go:
As I read Hebrews, the themes seem Pauline. The big pictures remind me of Romans, Galatians, etc.
However, the language seems fairly complex, like Luke would write. The wording reminds me somewhat of Luke-Acts.
My guess is this: Paul preached a series of sermons to a largely Jewish-Christian audience. Luke, sitting nearby, took notes on Paul's teachings. Later, Luke went back to his notes and wrote out in full what Paul said. So the themes are Pauline, but the language is more Lukan.
I freely admit that I'm no Greek scholar. Much of my conclusion is based on what I have heard from others. However, it seems to make sense in light of the data.
I'm glad we don't have to worry about the inspiration of Hebrews even though we are uncertain of the author.