I am not sure any work's essential theme can be summed up in such quick detail. You might have withstand a bit more of an explanation in order to better understand the work, in general. I think that the when examining the theme of the work, one has to look at Koly. The level of change she undergoes from the start of the work to the end of it could represent the notion of character evolution. The empowerment and sense of personal understanding that she undergoes could represent a theme. Along these lines, the idea of being able to assert one's own sense of individual identity in a community setting could also be another theme, as Koly learns to embrace her own voice in a traditional setting.
Well the theme that mostly sums up the whole book is obviously Prejudice. How prejudice is displayed throughout the obstacles Koly has to cross. Prejudice between being a girl or a boy in India. Girls aren't thought of much and are basically treated by slaves by their in laws while the Sons are pampered and spoiled.