Automatic dark theme switching

Not sure if this is a phosh issue (ie is it something handled by the compositor/display manager, or is that automagically handled by the services in GNOME) but I expect the phone to shift its screen towards the reds to reduce blue light at night, AND to switch to a dark GTK+Qt theme, so that it's 10x easier on the eyes.

Assuming there is an ambient light sensor, it should also switch to dark theme variants whenever the user is in the dark.

• Automatic red shift (by time)
• Automatic theme switching (needs to go into g-s-d)
• Redshift/Theme-switching by environment brightness (needs to go into g-s-d)