I would like to change the sky or give to fallout 3 another atmosphere, but Im not intrested in a weather overhaul, very dark nights and lights in the streets or such things.

http://www.fallout3nexus.com/downloads/file.php?id=6550 is likely the closest you're going to get with your requirements. It's got some options, and spinoffs.

There are others, but these cover exactly what you asked for:

http://fallout3nexus.com/downloads/file.php?id=8069
http://fallout3nexus.com/downloads/file.php?id=6550

Thats just what I was looking for.

Thanks!