This movie 's central underlaying theme is... (Contains Spoilers)
Happiness
One day Borgman tells the children the story of the white child that must face the beast that guards the golden key to happiness. Ironically he is that beast. That beast that shows up at their door one day unsolicited and manipulates it's way into their lives by feeding off their every weakness. In order for the family to hang on to their happiness, they need to be vigilant and strong.
At one point we see Richard's wife break down in tears saying that she feels undeserving of the happy life they have. So the theme of happiness is ever-present throughout the movie and I believe it to be the main theme.