Sorry if this sounds a bit rude, but rabbits are Crepuscular, in that they are most active at dawn and dusk, in low levels of light. They also spend a great deal of time in burrows underground with no light. Their eyes are very excellent at seeing in the dark (remember being told to eat your carrots to get good night vision?), and they tend to rely on hearing and smell more anyway. So no, I highly doubt that rabbits can be afraid of the dark; if anything, they feel safer from predators when it's dark.