The Moral of the Story

So, I've decided to do Mayflies for my two-year-novel course. Writing down the basic idea and genre were easy enough. This week, however, the task is to assign a theme. I've put some sort of thought into it before, but I'm entirely new to trying to incorporate a theme from the beginning rather than writing the story and seeing what comes out of it.

I started out stating the theme as injustice, and how it may always be present, but that's no reason to give up on justice. One observer suggested it would be better to call the theme justice since no one is seeking injustice. Another commented that destiny could work well, too.

Are there any suggestions from readers here? Any suggestions on writing in themes in general? I could use some assistance.