There are several themes that Mark Twain explores in his wonderful novel The Adventures of Tom Sawyer. Adventure is certainly one of the main themes, and Tom and Huck and their friends enjoy more exploits than any kids their age should ever experience. Friendship is a major theme, since both Tom and Huck feel like outsiders in their own families. Since neither of the boys are happy at home, their bond becomes one of the most famous in all American literature. The boys' wild imagination is evident throughout, and they manage to maneuver in and out of trouble through quick thinking and unexpected turns. The boys' loyalty toward one another is another theme, and both of them trust each other more than their families or other adults. Superstition also plays a big part of the story, and there are many examples of 19th century folk tales and notions to be found. Truth vs. falsehood is another theme that runs rampant through the story. Tom tells many lies and whoppers, but he also has a conscience, and there are times when he realizes that the truth must be told.
The idea of adventure is central to this novel in a number of ways, as pointed out by bullgatortail. Adventure, in The Adventures of Tom Sawyer, is both a literal and a literary theme.
Tom Sawyer is a thrill seeker, driven by imagination and a highly specific sense of nobility. The fact that he experiences some very high-stakes situations is a by-product of another fact -- his quite literary imagination. The novel's title is the first indication of the referential nature of the book, as Twain implies with the title a knowledge of the adventure story as a genre and, more subtly, suggests that his book will participate in that genre in a tongue-and-cheek manner.
Repeated references to other tales of high adventure demonstrate the narrative's awareness of the adventure genre.
"Injun Joe's cup stands first in the list of the cavern's marvels; even “Aladdin's Palace” cannot rival it."
Additionally, the novel is indeed crafted with many references to the author (Twain) and to the fact of authorship in ways that create a basic sense of contingency or ironic self-awareness throughout the text. This is a story being told that knows it is a story being told. This self-reflexive style is nicely aligned with the sarcastic tone of the novel and the overt social commentary offered in the text.
Tom's adventures, given this stylistic context, become part of a commentary on adventure stories. More specifically, Tom and his story come to make a comment on the role of imagination in the very premise of adventure -- the particular sense of put-on nobility of romantic "swashbuckling" that connects Tom Sawyer to Don Quixote, another ironic hero.
As a story-teller and as a boy enraptured by tales and folklore, Tom is the inveterate self-made man, almost as much made up as he is real, very muck akin to Don Quixote of La Mancha.
Tom is introduced as a liar and story-teller and lover of fiction, as we see here in a passage from the first chapter.
"[...] and Tom said:
“You're a coward and a pup. I'll tell my big brother on you, and he can thrash you with his little finger, and I'll make him do it, too.”
“What do I care for your big brother? I've got a brother that's bigger than he is—and what's more, he can throw him over that fence, too.” [Both brothers were imaginary.]
“That's a lie.”
The coupling of imaginative power (e.g., untruth) and action can be argued to characterize the nature of this novel. The resulting thematic suggestion is that adventure is not only what might befall an individual. Adventure is a state of mind. Taken as a thematic idea, this view of adventure goes a long way to explaining the charm of the novel and the sly, referential nature of the text as well.