Imagine yourself in a well-lit room. This room is connected to another room by a door. This other room is completely dark. You open the door connecting the two. The dark room gets lighter as the light rays stream in. The light room does not get darker.
So it goes with us. Light overpowers dark. Dark has no rays. It is but absence of light. The only way for this lit room to get darker would be if you were to cover the globe from which the light rays come.
The only way darkness can overcome the Inner Light is if we artificially obstruct it with imaginary thoughts of gloom. If indulged in by the ego, emotion becomes less of a gift but simply an obstruction from the Light.