Firstly, let’s settle a question that may make this easier to understand: Are theme and premise the same thing?
For an answer, let’s look at a couple of examples from Rob Parnell’s article “Theme and Premise — What’s the Difference?”
* Take Romeo and Juliet. The premise is two young people from warring families fall in love. The theme is star-crossed love leads to tragedy.
* Pride and Prejudice: the premise is that a feisty young woman needs to find a husband. The theme? Love conquers all.
* And my own example from Lord of the Rings: the premise is that a young hobbit finds himself in possession of a ring that can destroy the world. The theme, in my opinion, is courage and self-sacrifice for the greater good.
So our answer is that theme and premise are not the same. Premise is the situation that starts the story, what it’s all about; theme is the subject of the work, the heart of the story.
It’s a chicken and egg question. 
What comes first, the story or the theme? My definitive answer is, it depends. There are relatively few original story themes, but as writers, we may not recognize at the outset that we have a particular theme in mind.
Let me ask you a question: what’s the theme of your life? Unless you’ve recently done this exercise, you might not be able to pick it out. You’ll need to go back and consider who you are and what you’ve accomplished, how you’ve lived. In other words, you’ll need to review your story.
I think writing is similar. We need story before we can pick out a theme. The theme may be in the back of our minds, mulling around as we develop characters and motivation and conflict. It may be may be an age-old concept but it only comes through once we’ve brought it to life through the story itself.
For more on this “theme,” check out
Theme to Story http://learnedaboutwriting.blogspot.ca/2008/06/revising-novel-theme-to-story.html (by whoever used to write My Writing Life).