Actually, the manual (Z110 - pg 158, Z120 - pg 167) says you can adjust the brightness, contrast, color, AND Dimmer.
"The adjustments of Dimmer is stored separately when your vehicle’s headlights are off (daytime) and when your vehicle’s headlights are on (nighttime). Dimmer is switched automatically only when the value of nighttime is lower than the value of daytime."
To get into the Picture Adjustment mode, "Press and hold the MODE button. Press and hold to display the “Picture Adjustment” screen."
Haven't tried this, but it appears that it allows you to customize the brightness for both daytime and nightime (based on the ILL sensor/circuit) display.
BTW, this question, and my response, are way off-topic. It doesn't even belong in the "Hacks/Mods" section.
Update: I just noticed that the OP did ask about brightness adjustment at night, so this wasn't technically "Off-Topic". However it isn't a hack/mod either, so that should have been posted in either the General or Problems/Troubleshooting forum.