I gather that a theme is a premise, but a premise isn’t always a theme. A premise describes a way in which the story world works. So then I think the biggest theme is the highest abstraction of the way in which the story world works.
For example, in this story ravens are better at carrying messages than pigeons. If I happen to be a pigeon expert in real life then maybe I will have more difficulty with suspending my disbelief every time a raven carries a message from one castle to another. But most likely, I won’t have any or much problem taking the story seriously just because of the raven thing.
The author is pretty good at choosing the ways in which he can make his world different and more fantastic than the real world without too harshly offending too many readers’ capacity to take the story seriously.
Another and perhaps more consequential premise is that, in this story, there are giant lizard-snake-bat animals who can spit fire. That premise has a pretty explosive effect on how the story unfolds. And I’d wager that it offends more peoples’ capacity to take the story seriously than the raven thing.
And another perhaps even more consequential premise is that, in this story, revenge always bites back. But that one’s much more difficult to prove than those other premises. What exactly counts as a bite? And how much time is allowed to pass between the revenge and the bite before we’re no longer justified in assigning causality to the revenge? Well, I’ll have to pay close attention to the characters and how events unfold, and to remember to reserve judgement of the beginning and middle of the story until I’ve seen the ending.