Lights on the stereo are usually hooked to the dash lighting circuit, so it's not at a constant "bright" level.
The new stereo MIGHT draw quite a bit more power for it's lighting than the previous unit.
Does the stereo lighting work properly with the dash lights?
If so, and IF you get adequate lighting with the new setting, there may not be anything "fixable".
Otherwise it's time to question the installer. They did do something related to dash lighting, and you don't want lights so dim they can't be seen properly.