The theme of The Titan's Curse would be friendship and loyalty. In the book, Luke and Atlas took Annabeth, but before Percy could do anything else, he immediately went back to Camp Half-Blood to get help. Zoë Nightshade, a hunter, also did something similar when Artemis was captured. Zoë sought help and started to search for Artemis once she found out. Friendship is shown by how Zoë, Percy, Grover, Thalia, and Bianca stuck together in the story in search of Artemis and Annabeth.
Loyalty also plays a big roll in the theme. For instance, when Bianca sacrificed herself to save her friends from the metal giant in the Junkyard of the gods. Even though Percy warned her that it was dangerous, and he basically forbade her to go, she went anyway, putting her friends before her. Loyalty is also present in the book when Thalia had the choice of having all the power in world by sacrificing the Ophiotaurus, or staying by her friends and finishing the quest. It took a lot of courage and strength to pick between those choices.
Loyalty also plays a big roll in the theme. For instance, when Bianca sacrificed herself to save her friends from the metal giant in the Junkyard of the gods. Even though Percy warned her that it was dangerous, and he basically forbade her to go, she went anyway, putting her friends before her. Loyalty is also present in the book when Thalia had the choice of having all the power in world by sacrificing the Ophiotaurus, or staying by her friends and finishing the quest. It took a lot of courage and strength to pick between those choices.