Integrity
Perhaps the most important theme in Ahimsa is standing up for what you believe in. In the novel, Anjali and her family gave up quite a bit to stand up for Gandhi's movement of ahimsa and against the oppressive British rule. They gave up their expensive, cherished possessions (like foreign-made clothes) and their comfortable life to fight for something that they believed in.
Few people - especially those with some financial wherewithal - are willing to life a finger to stand up for change. Anjali and her family went above and beyond their call of duty to fight for something that is just and right and good.
Breaking social norms to be more successful
Another major theme in the novel is breaking social norms to be more successful. In the novel, Anjali and her family are portrayed to be in an upper-level caste (a system in India that divides people by class). However, to be more successful in their attempt to further Gandhi's movement of non-violence and to expel the British from their country, Anjali and her family (as well as other Indian families) interact with lower castes. Specifically, they interact with people in the "untouchables" caste, the lowest caste someone could be in India.
In doing so, Anjali and her family had to humble themselves. Their culture dictated that they were prohibited from even interacting with the so-called "untouchables." In talking about the "untouchables" the family broke cultural norms so that they could do what they needed to do.
Passing the mantle through multiple generations
In the novel, after Anjali's mother is jailed for her work in Gandhi's movement, Anjali herself takes up her mother's mantle and continues her work in the movement. Anjali does so out of familial obligation and because she, too, believes that the British need to let Indians govern themselves. In other words, Anjali makes the choice to take action against the British because she felt some sort of obligation to do so.