Social class is a very important theme in the book, as it is in all of Jane Austen's works. Regency England was a very hierarchical society with clear boundaries between the respective classes. And it's clear from reading Emma that Jane Austen firmly believed in maintaining those boundaries. At the same time, she insists upon the importance of the upper-classes' responsibilities towards those poorer and less socially prominent than themselves.
We see this in Emma's well-meaning, but ultimately misguided attempt to instruct Harriet Smith in the ways of Highbury society. It becomes painfully obvious that Harriet doesn't really belong among the upper echelons of society; she is hopelessly out of her depth, artificially elevated to a higher station by Emma's assistance. Thanks to Emma's ill-judged intervention, Harriet gets ideas above her station, developing unrealistic expectations about her future marriage prospects. Emma should have learned from the example of Mr. Weston. His first marriage was to a woman of a higher social class, and her inability to adjust to a lower standard of living was a major factor in their unhappiness together.
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.