Hey there!
The theme of Cinderella in my opinion, is "good outcomes happen to those who work hard."
Cinderella was undoubtebly put in a bad situation. She had to work everyday for nothing, and watch her stepmother and stepsisters torture her while they went to balls and parties. Additionally, her father and mother died, so she must have been overwhelmed with grief, too.
But she kept going, and she moved on. She didn't stop, and one day, her opportunity came and she took it in an instant. She was able to marry the prince, and being a good person she gave her family a second chance- because everybody deserves one.
Hope this helps!