I suspect others have punctuated this differently. Some editors probably make this two sentences. It could even be three: O. My son. What theme?
I might go with O, my son. What theme?
But the Queen’s question really is a sensible one, in fact. And she is really listening to him even though he is not making a lot of sense.
I mean, once we know what he means, what the theme is, more or less, then it’s kind of logical – but backwards. As most of us are, really. Logical – but only backwards. What is mysterious in one direction can sometimes add up in the other.