Sky: Darker, bluer sky and improved horizon haze at night

Add new colours 'skycolour_bright_night', 'bgcolour_bright_night'
and enable these between sunset end and sunrise start
Night sky has same hue as day sky but is darker and more saturated
Night horizon haze (bgcolour) is slightly less saturated and
slightly brighter than night sky, to be consistent with daytime
horizon haze