I would say that the theme reflected here is the theme of freedom. Much of this book is about the desire to be free -- free to do what you want with yourself.
When Huck escapes from his father, he is setting himself free. He sets himself free from the demands of his father and the demands of society (like the Widow Douglas).
At almost the same time, he meets Jim. This shows another aspect of the theme of freedom as Jim tries to keep his freedom and Huck struggles with what to do about Jim -- give him freedom or obey society's rules.