If it's an lcd monitor, you could try replacing the inverter and ccfl. If the ccfl is getting "old" it will lose some of it's luminence and tend to leave the background dark.
Check for a manual for your monitor and see if you can find specs for the inverter (changes low voltage to high voltage) which is the most likely issue. If you can find both the inverter and ccfl (cold cathode fluorescent light), replace both and you should be good to go.
Mark