Why don't the home screen keys at the bottom illuminate

I'm wondering why the bottom 3 touch screen keys don't light up. There's the circle in the middle, which is the home key. Then there's a dot on the left, which is the back key, then there's one on the right that's the open apps key. There's no option to turn them on and I don't have the screen savor turned on. I'm thinking they probably just don't light. I can use them perfectly fine with out them lit up, but it would be much better if they did light. Thank you! I'll appreciate any help you cam give