Acceptance
Stuart Little "arrives" at the Little household as the Littles' second son, and even though to those of a more realist, pedantic mind this is an absolutely ridiculous, insane thing to have occurred. The Littles by all accounts should have considered it freakish and/or miraculous, but how they actually respond carries with it a primary theme of the book: they respond with unfettered acceptance and love. They do not shun their tiny son, and they do everything they can to accommodate him. It is heartwarming and comforting how his difference does not matter to them – he is their son, their child, their pride and joy, and nothing will change that.
Everybody Grows Up
Stuart starts out small and remains that way. Just because he perpetually carries the look of a child, however, does not mean that he remains a child. A significant theme in the book is that even the most dependent and vulnerable of children still must mature and this can be especially difficult for others who continue to view their emotional, mental, and intellectual growth as being as stunted as their physical development. Stuart’s maturation into having a job, driving a car, and falling in love is painful and an uneasy transition for everyone.
Bravery
Stuart is tiny and navigating the world is hard, but he is undoubtedly and unequivocally brave. He sails boats on the pond, confronts Snowbell, and decides to venture out to find his true love, Margalo. He does not let his small size or other people's lack of confidence in him deter him from both doing what he loves and from doing what is right.
City vs. Country
Stuart is a city boy, and the way he dresses and comports himself reveals him as such. Margalo, by contrast, is a country girl, on leave from a beautiful place far away filled with fields and flowers. In the first part of the novel, Margalo comes to the city and sees Stuart's world, and in the latter Stuart ventures out into the country to find his bird. Margalo finds much to interest her in the city and Stuart enjoys the leisurely pace and beautiful scenery of the countryside. White does not suggest that either place is better (though he famously wrote of his love of New York), and that experiencing both enriches one's life.
Fortitude and Endurance
Most things in Stuart's life are admittedly difficult due to his size, but he is tenacious and fortitudinous. He refuses to give up, and instead asks for help or devises ingenious solutions to his problems. Additionally, searching for a bird without knowing where the bird actually is from (or even what kind of bird she is) is an immensely difficult task, yet Stuart not only bravely embarks upon it (see theme above) but refuses to give up when it gets difficult. Even though at the end of the book we do not know if he ever finds Margalo, we can at least guess that he will continue searching for her until the bitter end.
Socialization
Once the fact of Stuart’s remarkable situation is accepted by the Littles, they can get down to the real business of dealing with such a unique circumstance: assimilating Stuart into the normalcy of everyday life. Due to the unique—even bizarre—reality of Stuart’s differentiation, socialization essentially means making adjustments to the reality of daily life to conform to Stuart’s special needs. And there, of course, is the real crux of this thematic dimension of the novel: Stuart is, in reality, a special needs child and the Littles are ideal parents for him because, in a world that demands conformity, they are committed to conforming the world to fit Stuart’s requirements. At the same time, however, they are equally demanding that Stuart accept and follow the path of his human-ness and repeatedly attempt to influence him away from exploring his mouse-ness. As in real life, socialization is a prickly concept in the world of the Littles and their special son.