Of course, discussing the "main theme" of any novel is a task fraught with problems, as it is very difficult to decide on the "main theme" of any text, as it is a highly subjective question. However, for me, in this novel of Austen's, one cannot escape the way that the plot and characters lead towards a message about growing up and marriage. We are presented with a Miss Emma Woodhouse who at the beginning of the novel is immature. She makes many mistakes; she shows herself to be a social snob; she meddles unnecessarily and intrusively into the lives of other people with tragic consequences. However, partly because of some of these mistakes and the humiliating and painful consequences that she suffers, and partly because of the patient and loving guidance of Mr. Knightley, we see a very different Emma at the end of the tale. Emma grows up throughout the novel more and more to have a self-understanding about herself that is shown to yield maturity. It is only when she reaches this state of being self-aware, and of course part of this self-awareness is the realisation that she loves George Knightley, that she is shown to be ready for marriage.
Thus the novel has much to say about the process by which we mature, self-knowledge and how these two concepts are linked to marriage.