Mark Twain's The Adventures of Huckleberry Finn ,follows the narrative tradition and style of the maturation novel, also known as bildgunsroman, which is a coming of age novel.
This being said, the implied theme of The Adventures of Huckleberry Finn is the analysis of society through the eyes of a maturing child within chaotic and dynamic situations. When we hear that the narrative is a series of adventures we expect for a lot of activity to take place, and a lot of problems to be resolved. This is why Huck's view of society as a boy who is growing and maturing is so important to these adventures: Because the theme of the story implies that he will learn a lot from these experiences.
Meanwhile, we have the topics that pivot Huck's adventures. The most permeating topic, of course, is slavery, racism, and the hypocrisy of Southern societal values. We see these in the treatment of Jim and the language that is used in general to differentiate people within society. We see it in the repetitive, yet empty, traditions of church and prayer, and we also see it in the superficiality in treatment from one person towards another.
Another huge theme in the story is innocence versus reality. Huck, through his maturation, sees things with more spark and excitement than what they are really like. He overrates people, fantasizes about the lives of others, and imagines greatness where there is not. He also lives oblivious to the needs and sacrifices of Jim. As the story progresses we sense a much more knowledgeable and "seasoned" Huck which will eventually see life with much clearer eyes.
Additional themes include abandonment in the form of Huck's alcoholic and terrifying father, Huck's need to free himself for an intrusive and demanding family, Jim's own sad situation as a slave, and the overall dangers of life in the Mississippi.