A nice star cubemap, make up most of the athmosphere.
For the ambience, try to make something like
10,10,20 For RGB Color (Blue is always a nice night-time color)


Some small recent games of mine:
Griefer 4k http://damocles11.byethost9.com/griefer4k.html
Kobold 4k http://damocles11.byethost9.com/kobold.html
Cave 4k http://damocles11.byethost9.com/cave4k.html