Since firmware version 20130515, the LCD contrast is controlled by the firmware. Prior to that it was just left to whatever that LCD had internally and it varied a bit. Still, each LCD is slightly different and the new default settings may not work well with all LCDs.
If the display is too dark or the background is gray after updating the firmware, try changing Settings->Display->Bias Mode from 4 to 3. If the display is then too light, try decreasing Settings->Display->T Coefficient