It was the best of times, it was the worst of times. It was about the disorder of the French Revolution, it was about the contrasting order of England. Dickens only wrote two novels that are considered "historical fiction" and this is one of them. In the novel and historically, many French aristocrats escaped the madness that was France to emigrate to England until things settled down. It took a long, long time.
The novel, then, is in many ways the story of history itself which is a constant cycle of life and death, good and bad, order and disorder, sadness and happiness, love and hate, condemnation and redemption. There are many symbols, motifs, characters, etc. that revolve around these opposing forces. If you go back over the novel, you will see this. I believe that Dickens was trying to point out that life is like history - with the same opposing forces -- but that ultimately, love wins out.
You can read about the themes here on eNotes.