Well, in this novel, the theme of discovery is prominent. The characters constantly discover new species, new underwater landscapes. There is also the theme of confinement. Although they are in a large submarine, they are still confined to a limited space in the vast ocean. And a theme of wonder, as the descriptions of the underwater world evoke a sense of wonder in the readers.
The themes include exploration. The characters explore the vast and mysterious underwater realm which was largely uncharted at the time. Morality is also a theme. Captain Nemo's actions, sometimes vengeful and other times heroic, make the readers question what is right and wrong. And there's the theme of the relationship between man and nature, as they are immersed in the beauty and danger of the ocean.