Due to the sheer size of this novel, it will be difficult to provide specific examples to such a broad question, but perhaps consider the following ideas to get started.
One major theme of this book is justice vs. injustice. There are several scenes which take place at night where illegal activity is going on and someone is witnessing it. Most of these scenes include M. Thenardier, though sometimes he is under an assumed name. Perhaps you could examine the role that night plays in cloaking injustice. This also has a Biblical implication as well. Evil deeds are associated with darkness while goodness is often reflected in light. Les Miserables has been discussed as a novel reflecting the Biblical virtues of forgiveness and redemption, so it would not stray from the overall purpose of the novel to look at darkness imagery through this Biblical lense.
Examining the theme of forgiveness and redemption, darkness imagery could also be analyzed through Valjean's conversion. Notice his actions done in darkness. Stealing the silver first, and then fleeing. Later, with Cosette, before he has established himself as the Mayor, he frequently flees and hides at night as well. There is also a mental or emotional darkness associated with Valjean's denial of his true identity. Again, this seems to point to the idea that darkness encourages deceit (or evil), and the truth is reflected in light.
Between these two ideas I think there is more than enough material for an essay. In fact, I would recommend choosing one over the other rather than attempting to incorporate both into the same essay. I think both ideas are broad enough in themselves that they need not be combined.