Themes also emerge in nonfictional writing, particularly history and biography, in which the themes the author chooses to pursue will powerfully influence both which facts and events are more heavily weighted and the author's interpretation of those facts in portions of the narrative in which it is appropriate to present the author's educated guess as to why or how something happened or someone acted as they did. (Whew! That's a mouthful.) The same facts that lead one author to craft a narrative showcasing the theme of perseverance against the odds might lead another author to explore the question of "what is social justice?", resulting in two quite distinct accounts of the same event or person.
Authors, by the way, are under no obligation to answer the questions presented in their themes and are often better when they do not, allowing the reader's mind to grapple with the question and continue exploration of possible answers even after the journey of the book's protagonist has reached a conclusion. A question that leads to a pat answer is more a moral than a theme, and while morals have their place (a la Aesop's Fables), they seldom make for the kind of books that provoke deep thought or become old friends, kept for re-reading and re-exploring.
Because my early concepts of the tale of the Firebird revolve around a tension between two worlds and two modes of existence that the main characters are caught between, the primary theme that seems to be emerging is this: What does it mean to be human? That is a question with so many ramifications and sub-themes that I think I'm in little danger of becoming trite, at least. It's also a question that to some extent makes me glad that I'm not trained as a philosopher, as I'd rather not fall off the other side of the horse into being ponderous or abstruse and so risk losing the story.
More themes may start developing as I proceed, for a book seldom has only a primary theme and nothing else; secondary themes will almost always come out to play as well. But at least I have some idea of which way the river of my thoughts is running.