Someone correct me if I'm wrong, but I do not believe there is any sort of brightness or contrast control for the LCD on these keyboards, as there is no such control on my PSR-E433. Is this a keyboard that you've had for a while, and you're now noticing this dimming screen that was not there before, or did you recently get this keyboard and just feel the display is kind of dim? Because if you just got the keyboard, it is possible that this is normal. I know the angle that you are viewing the display can make a big difference on the appearance of the readout.
However, if you are now noticing this and it's a keyboard you've had for a while, then it may just be a display that is going bad. The only other possibilities that come to mind have to do with powering the keyboard -- if you're using batteries and they're getting weak, you might get such an issue. Or, if you're using a power supply and it is going bad and not supplying proper voltage and/or current, then that may also cause a dimming display. However, I'd think you'd also notice other issues, such as low volume or distorted sound, if it was a power issue. But it is something to consider.
EDIT: I just noticed all those other keyboards you have in your signature -- so I'm sure you've probably already considered everything I mentioned.