feature request: night mode

[hal80]

Hi, lot of devices nowadays have a night mode with reduced screen brightness and adapted colors. Missing that feature on zynthian. If PWM control (like suggested here https://github.com/zynthian/zynthian-hw/issues/6) is difficult, maybe we can start with a second user definable color scheme where the user can set night compatible colors (such as red, dark red, black).

[jofemodo]

From my POV the current color scheme works quite nicely at night, but i'm open to improvements, of course. Please, could you make a graphic proposal (scale 1:1, please).