Making Nights Darker
From Mod Wiki
Inside the file config\weathers\weather_default.ltx, there are a few things to explain first. The first section, [sect_default_weather], shows each hour and then refers to a weather setting later in the file to how the weather will appear. It is very straight forward. Now, to make nights darker, you need to go through the hours that it would be night, and change the ambient and hemi_color variables to somewhere around these:
ambient = 0.0025, 0.005, 0.0075 hemi_color = 0.025, 0.05, 0.075, 0.1
You need to be creative however, as it needs to slope from bright to dark to bright to make it look realistic. Experiment, not one setting will fit to what you want.