What is the main theme of the novel Great Expectations?

There are many themes in Great Expectations, but one of the most important is that of social class. At the time when the book was written, Britain was the most advanced industrial nation in the world. One consequence of this was greater social mobility, with a new class of businessmen, manufacturers, and entrepreneurs gaining greater wealth and power, many of them marrying into the old aristocratic families. Wealth, rather than noble blood, was becoming the most important factor in determining one's social status.
We can see this point illustrated in Great Expectations. Pip comes from a very humble background: he is an orphan brought up on the Romney Marshes by his sister and her blacksmith husband. Yet thanks to a generous bequest from Abel Magwitch, he's able to live the life of a gentleman among London's fashionable society. Magwitch, too, through his successful business, is enabled to transcend a life of extreme poverty and criminality and acquire considerable wealth. In a previous age it would've been unthinkable for someone like him to achieve such a position of importance in society.
Dickens often shows us the seedy underbelly of mid-19th-century Britain, with all its poverty, cruelty, and rampant hypocrisy. Yet in Great Expectations, he also shows the positive side, that the new society was better able to provide more economic opportunities for more people than ever before. What the new industrial society does for Pip and Abel Magwitch, it had also done for Charles Dickens, enabling him to rise from a poverty-stricken childhood to a life of considerable wealth and success.