What is the theme of The Odyssey?
One theme of this epic poem is that there is no place like home. Odysseus spends almost twenty years trying to get back to Ithaca and to his family, not content to be anywhere else (even if he's basically been in paradise twice with Circe and Calypso). He says, "Nothing more sweet than home [...] can there be, however rich one's dwelling far in a foreign land [...]."
Another theme addresses the value of life. When Odysseus is in the Underworld, he speaks with Achilles, telling him not to be sad about being dead because he is so incredibly respected by all the living. However, Achilles says, "'Mock not at death, glorious Odysseus. Better to be the hireling of a stranger, and serve a man of mean estate whose living is but small, than be the ruler over all these dead and gone." In other words, it is better to live a life of no consequence than it is to be powerful among the dead. Life is to be cherished.
Penelope's and Odysseus's commitment to one another and emotional fidelity (one might argue he didn't have a choice about sleeping with Circe and Calypso), at least, also says something about the importance of loyalty for a member of either sex. This constitutes another theme.
Further, Odysseus's interactions with Polyphemus, the Cyclops who ignores the gods' rules about hospitality because he believes he is more powerful than they, teaches the importance of humility. Polyphemus says, "'The Cyclops pay no heed to aegis-bearing Zeus, nor to the blessed gods; because we are much stronger than themselves.'" This proud belief comes back to hurt the Cyclops in the end when Zeus allows Odysseus to blind him. Odysseus calls the monster an "'audacious wretch'" for his pride.