Imagination and the value of story
This story features the brilliant imagination of a young girl during Christmas time whose difficult life leaves her unable to sleep. At night, sometimes she wanders around her godfather's estate, and she imagines an entire world. The novel depicts her imagination as if it is really happening, but the reader should distrust the narrator's point of view about that, since her imagination seems real to her. That's one of the most important design features of the novel, that her imagination seems real, because to her it is true. In a way, they're more true in a greater sense than if they had been literally true. When she creates stories from her imagination, she learns from them authentically. With this domain of play, she can work through the various pains and fears of her blossoming mind.
Time, fate, and the loss of beauty
Marie's young imagination becomes fixated early on with the passage of time and the inevitable loss of beauty. She is trying to reconcile the fact that as time passes, people age and their faces change, people get sick, and people even die (like her parents, perhaps; after all, she is at her godfather's estate). In a word, she is concerned about the decay of the world toward some tragic end. Two major plot moments highlight this theme: The curse of Pirlipat where she goes from being beautiful to being hideous, and the death of the nutcrackers that started the war with the mice in the first place. Notice how the toys are human inventions (carved and painted), but the mice are agents of nature. That "man versus nature" conflict is also relevant to this theme.
Loneliness
The unspoken theme that gives this novel its melancholic feel is that no one believes Marie's imagination. Not only is this tragic because Marie isn't taken seriously, but it's also a tragic indication that the person taking care of Marie is not comfortable or familiar with the way a kid's mind works. Marie doesn't need Drosselmeyer to believe her literally—she really just wants to talk about her emotions with someone, and telling him about her games is an attempt to be a little less lonely. When he thinks she's just being silly, he rejects her further. These are all ways that Marie deals with loneliness, and perhaps she is even mourning. Her imaginary stories are dark and violent, and often the dreams involve a community rallying to support each other in the face of trauma and pain—which would be very desirous to Marie.
