There are are a number of themes present in The Adventures of Tom Sawyer. If you need to focus on just one main theme, I would suggest examining the idea of the unbridled joys and innocence of youth. Throughout the story, Tom frequently gets into all sorts of mischief which can be chalked up to his youthfulness. He is carefree without the burdens of adulthood. As a kid from a town, he is also free from the responsibilities that his peers living on farms would have had at the time this story is set.
Tom's youth imparts him with an active imagination which is praised, despite the shenanigans that it gets the title character into. Throughout this story, Twain explicitly focusses on the joy of a life free from the responsibilities of adulthood and the possibilities and potential of unbridled youth.
Mark Twain is deliberate in making sure that this is a story of a childhood, not a coming of age story. The story does not end with Tom coming to a certain degree of maturity, although he does learn much during its telling. In the conclusion, Twain acknowledges this by stating that if he were to continue the narrative any longer, it would cease to be a story of a boy and become one about a man.
By writing about Tom Sawyer, Twain is in many ways reflecting on his own childhood and drawing parallels with any child reader of the book. Twain may be reminiscing about his childhood days, but Tom does not since he is still experiencing them. In this way, adult readers and child readers of this story experience it through different lenses.