What,according to you, is the underlying theme of Twilight?

If you're forced to move in with your father, you may just end up with a family of vampires, who are hated by your bff werewolves, but it'll work out because one of them will fall in love with your daughter. ._.

Life choices/free agency.
You don't have to accept or take what's handed to you. You always have the option to make what you want happen.
The Cullens chose to deny their nature and have high morals.
The choice to resist temptation.
Bella chooses Edward, despite the odds being stacked against her.
And so on.

The opposite is demonstrated with the Quilieutes (I always spell that word wrong). I think it adds to the theme, showing what life would be like if you didn't have many choices.

That's the main one, I think.