What is the theme or main idea of The Adventures of Tom Sawyer by Mark Twain?

The main theme of The Adventures of Tom Sawyer is the joys of childhood when the world is full of wonders and children are free of the heavy responsibilities of adulthood. Mark Twain was writing about his own boyhood, growing up in the small town of Hannibal, Missouri, right on the edge of the great Mississippi River. Tom is a town boy. If he lived on a subsistence farm like the majority of Americans in those times, he would not have the freedom he enjoys in this novel. Farm boys were put to work at an early age, and much of their work consisted of plowing behind a horse or mule. They were often called "plow-boys." American literature was full of stories and poetry about boyhood. A good example is "The Barefoot Boy" by the eminent poet John Greenleaf Whittier, which begins:

Blessings on thee, little man,
Barefoot boy, with cheek of tan! 

And contains this sentiment:

From my heart I give thee joy,—
I was once a barefoot boy! 

One of the most prominent authors of the time was Booth Tarkington, who wrote two very popular books about boyhood: Penrod, and Penrod and Sam.

The theme of The Adventures of Tom Sawyer might be best expressed by quoting from one of the most popular poems of the time, "Ode: Intimations of Immortality, From Recollections of Early Childhood" by the great English Romantic poet William Wordsworth.

Thou little Child, yet glorious in the might
Of heaven-born freedom on thy being's height,
Why with such earnest pains dost thou provoke
The years to bring the inevitable yoke,
Thus blindly with thy blessedness at strife?
Full soon thy Soul shall have her earthly freight,
And custom lie upon thee with a weight
Heavy as frost, and deep almost as life!

This is the spirit and theme of both The Adventures of Tom Sawyer and The Adventures of Huckleberry Finn. Youth is the best time of life because we are free of responsibilities and cares, and the world seems full of wonders and possibilities. We are immortal because we haven't yet found out we are not. The character Tom Sawyer became so popular that Mark Twain wrote two more novels in which the boy is featured: Tom Sawyer Detective and Tom Sawyer Abroad.