I have one:
Violence is caused by social class differences.
Here's my format:
subject + author's message = theme
The flaw with your theme is that it is not just the difference in social class which begets violence. That is certainly why and how it begins; however, we know that by the end of the novel the revolutionaries are starting to turn against each other, and Carton's vision of the future is historically accurate. Once the aristos are gone, the peasants fight against others of their own class. What that says, of course, is that violence begets violence. As others have said, there are lots of possibilities for effective theme work in this novel.
Bloodshed begets bloodshed. The terrible crime of the Evremonde twins begets the bloody revenge of Therese Defarge. On a larger scale, the oppression of the peasants--the Marquis alludes to the numbers of peasants slain by the family swords in his conversation with Charles, and the Marquis's carriage runs over a child--leads to the slaying of the Marquis d'Evremonde. On a still larger scale, the oppression and shedding of peasants' blood through killing labor and through starvation leads to the French Revolution.
There are many themes for this book. Dickens wrote A Tale of Two Cities for the purpose of bringing out the ills he recognized in his world. He chose to write historical fiction, about the French Revolution, to remind his people of a time in history when the abuses of the upper classes resulted in bloodshed.
Using your formula, consider Dickens's subject (the uprising of the poor in France) and his message (abuses of the poor can result in violence). The theme that results might be: If people suffer prolonged abuse, they will rise up to end their suffering.
Another subject is Carton's unrequited love of Lucie Manette. His message is that a person can make up for the wrongs he has done by giving up something. The theme is that redemption can come from sacrifice.
Hopefully you are able to follow this format to find some more themes! Consider the following subjects and what you think Dickens’s message is.
*Stryver takes credit for all of Carton’s work and he does not seem to care.
*Dr. Manette struggles to regain and keep his sanity because he loves his daughter.
*Madame Defarge is so desperate for revenge that she does not care who gets hurt in the process.
What themes can you generate from these?
Take a look at the link below for more themes.