What is the central theme of Oliver Twist by Charles Dickens?

The central theme in Oliver Twist is good versus evil. More specifically, Dickens explores the immense struggle that goodness must endure before it finally triumphs over its oldest and most bitter enemy. All of the good characters in the story—most notably Oliver himself—experience considerable challenges and hardship. Yet by remaining true to themselves, they are ultimately able to triumph over the forces of darkness.
Take the example of Oliver. Life's been a constant struggle for him ever since he could walk. A poor orphan forced to endure the harshness of the workhouse and its appalling conditions, Oliver's had the worst possible start in life. Given such an upbringing, we might expect Oliver to go on and pursue a life of crime. Yet even when he falls in with a gang of thieves, his innate goodness still shines through. He only commits crime out of fear of what Fagin and Bill Sikes will do to him if he refuses.
Although bad things may happen to good people in Oliver Twist, goodness wins out in the end, suggesting that it will always be much stronger than evil, however hopeless the odds may often seem.