Fancy, the theme may have nothing to do with the plot of your actual novel, but has more to do with the message you're trying to send to the reader with your story. I like Ember's suggestion, and it would probably fit for your story. But more generally speaking, your theme could be something like one of these:

- good always defeats evil in the end
- what goes around comes around
- people earn their just rewards
- love prevails
- hard work prevails
- perseverance prevails

Or you could send a message like one of these, depending on how you choose to end your story:

- the justice system never works
- life is unfair
- do whatever you want because you only have one life to live
- always look out for #1

You could choose any number of themes in addition to what I've suggested, and it's entirely up to you. But you can see how your theme might dramatically impact how your story ends.

Regards,
Michelle