Well, I take issue with any one work having a major theme, as you describe it. I think it is clear that any work opens itself open to a number of different possible themes and to identify one as "major" is to possibly ignore the full impact of the others. However, for me, one of the obvious themes is the way that this work explores the conflict between fact and fancy.
Fact is clearly the biggest component of the educational philosophy of Mr. Gradgrind, however, it is important to note how the cause of fancy is championed as being incredibly important, particularly through the character of Sissy Jupe. Consider how, throughout the novel, fact and fancy are set in opposition with each other, with Mr. Gradgrind championing the forces of fact and the circus folk, of which Sissy Jupe is clearly a member, marshalling the forces of fancy.
Also, let us consider the results of these two different approaches. Clearly Tom and Louisa, who have been brought up on Mr. Gradgrind's maxims of focusing on "nothing but the facts," show that they are able to integrate into society, although in different ways from each other. Sissy Jupe, on the other hand, although she has done so "badly" in Mr. Gradgrind's opinion, is clearly a child of fancy, and as a result is a much healthier individual who is able to care for and support Louisa in a way that neither of her parents did. However, note that facts are still seen as being important. Sissy needed the direction and guidance of Mr. Gradgrind, adequately yoked to her fancy to help her develop into a mature individual. One without the other is bad for anyone.