There are so many themes in this book, it is difficult to know where to begin.
I think love in all its forms, both tragic and blissful, is a major theme,
"Velutha smiled when he saw the Marxist flag blooming like a tree outside his doorway He had to bend low in order to enter his home. A tropical Eskimo. When he saw the children, something clenched inside him. And he couldn't understand it. He saw them every day. He loved them without knowing it. But it was different suddenly. Now. After History had slipped up so badly. No fist had clenched inside him before.
Her children, an insane whisper whispered to him.
Her eyes, her mouth. Her teeth.
Her soft, lambent skin." (10.256-259
Velutha's feelings toward the children change here. He has fallen in love with Ammu, and as a result, his love for her children deepens to the point that it takes him by surprise.
Here is another one about Love,
"Between them they decided that it would be best to disturb her discreetly rather than wake her suddenly. So they opened drawers, they cleared their throats, they whispered loudly, they hummed a little tune. They moved shoes. And found a cupboard door that creaked.
Ammu, resting under the skin of her dream, observed them and ached with her love for them. "(11.33-34)
A mother's love for her children can be overwhelming. The twins, in their innocence, are afraid that if they wake Ammu up suddenly, she'll have a heart attack and die. So they tiptoe around trying to wake her gently. Noticing this, Ammu is overcome with her affection for them.