Well, the themes are quite diverse. There's the theme of nature versus civilization. Huck feels more at home in nature, on the river, than in the civilized towns with their strict rules and false values. Another important theme is truth. Huck has to deal with lies and deception, not only from others but also within himself. And of course, the theme of social inequality is prominent. Jim is a victim of the unequal society, and Huck starts to see the unfairness of it all through his adventures with Jim.
One of the main themes is freedom. Huck is constantly seeking freedom from the constraints of society, like the rules of Widow Douglas. Another theme is friendship. His relationship with Jim shows a deep bond that transcends race at a time when slavery was still prevalent. Also, the novel explores the idea of morality. Huck struggles with the 'right' and 'wrong' as taught by society and what he feels is truly right in his heart, like when he decides to help Jim escape slavery even though it goes against what society says.