I don't think you can adjust the brightness when the displays are in "day" mode. There was a post on here about a way to force the centre display (at least the map view) into the night mode with the black background. I can't recall how you do it. I tried it and it did work, but I believe every time you shut down the car it would revert back to day mode and you had to do the settings change again, so not convenient.
