The older models of light covers will turn yellow with age and the heat of the light bulb. However, the most prevelent cause of the yellow is using a wattage that is way to high for the plastic.
The older ones were only rated for a maximum of 60 watts and we find a lot of people put in 100 watt bulbs, wich actually burns or melts the plastic light cover.
Make sure you have no more than a 60 watt bulb in there, and if the light cover looks unsightly, then throw it in the garbage.
The light cover is strictly cosmetic, cuts down on the available light, trapps the heat in there which causes the light bulb to burn out prematurely, and it looks unsightly.

The newer machines (within the last 8 years) have the ability to adjust the "time on" for the light bulbs.
The older ones are a set time.