The idea isn't so much light or dark squares per say, but the idea of superior influence in an area. It just so happens that, say an opponent with no light square bishop and no pawns on light squares may be permanently unable to contest light squares... maybe all over the board. In practice though it's more likely this superiority is just in one area. Anyway this means the opponent may use these squares as outposts to coordinate his pieces.
What I mean is, you may ask, if you're weak on the light squares, why not simply move to the dark squares and you're safe? It's because these light squares will be used as infiltration/outpost squares from where your opponent will exert pressure and can organize further attacks on all the surrounding squares.
Which squares are better, and how do you use this to your advantage?