Q. What is the primary theme of the Book of Ninevites?
A. There is no Book of Ninevites in the Bible.
Q. What role did Roman political and judicial authority play in the death of Jesus?
A. Who cares?
The Book of Jonah is not about the Ninevites. Obviously, they play a role -- a larger role in Jonah than the Romans do in the Gospels -- but they aren't really important to the story. Nineveh is just a place whose proverbial wickedness warranted a prophet to preach against it. It's the prophet's reactions, both to his call and to God's response to the Ninevite's conversion, that convey the spiritual senses of the book.
I'm starting to suspect the same is true of the Romans who crucified Jesus.
There's been a lot of talk lately on the question of "Who Killed Jesus?" Much of what I've seen strikes me as a fundamentally silly discussion: people saying, "You know, it was actually the Roman who crucified Him," with the sort of wide-eyed who'd've-thunk-it also used with, "You know, the Bible doesn't actually say there were three wise men." (This is opposed to a not-at-all silly discussion on the fact of human deicide.)
I'm starting to think the role of the Romans in Jesus' crucifixion was, from the perspective of the New Testament writers (and therefore, of the Church in the First Century), primarily a historical accident. Yes, there is theological meaning in the exercise of authority depicted, but mostly the Gospels record Roman involvement as a matter of historical, not spiritual, fact. If it hadn't been Pilate acting for Caesar, it would have been Herod, or Philip, or some other political power acting for itself. The New Testament is concerned, for the most part, with things far more important than political power.
Revelation is about God's relationship with man, in particular with His people Israel and those who come to know God through He Who is Israel's Glory. Pilate's sin was a failure of natural justice; we don't need Revelation to teach us about natural justice (although it helps). The sin of the one who handed Jesus over to Pilate -- a sin greater than Pilate's -- was a failure of faith, and teaching faith in Jesus Christ was the reason the Gospels were written.