What are the themes in the book The Adventures of Tom Sawyer?

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:

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“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.