In 'The Phantom of the Opera' novel, a major theme is the power of music. It has the ability to both soothe and manipulate. Christine's connection to music is what initially draws the Phantom to her. There's also the theme of isolation. The Phantom lives in isolation in the catacombs beneath the opera house, which shapes his character. And the theme of redemption can be seen as the story progresses, as the characters face their own demons and strive for some form of salvation.
Well, the themes include love, for sure. The love triangle between Christine, Raoul, and the Phantom is central. Then there's the theme of identity. The Phantom is a figure who struggles with his own identity due to his deformity. And the theme of art is also there. The opera setting and Christine's singing are important aspects related to art in the novel.