I've recently been reading articles about themes and such, and it made me wonder, what would you say the themes of the Valhalla trilogy are?

I’d say there are bits and pieces of things but the series is written to be pure entertainment, it doesn’t have a theme at its center. What there is: The structure of the world is fragile, and what we have with the people we care for is tough, but malleable. We can lose everything fast, so don’t fuck shit up.