One of the primary themes presented in this book is accepting diversity. Stuart Little is a mouse, in the book born into a human family who accepts him and makes adjustments for his smallness. There is an issue of the differences Stuart has compared to his human family. These differences can become an obstable since George is a little disappointed by his brother who can't really be the playmate that he was expecting. So George has to accept his brother for who he is and this is done through Stuart's remarkable personality.
Stuart is very courageous, he is not afraid to charge into difficult situations, like getting his mother's ring out of the drain. He races a boat in Central Park, crawls inside the piano to fix the keys for his brother and decides to look for Margalo, setting out to find her without any thought for his own safety.
Stuart can be viewed from the Little's perspective as a special child, and it is often said that special children born into a family may look different, act different, but are always a special gift. Stuart symbolizes this because he is so extraordinary that it is easy to overlook the differences that he has with his human family.