Specifically STEAM related.

I'm running a 100 tick zombie server on CS: Source and everything works fine. But all the maps are so bright, I'm currently shifting through darker maps:
cs_havanna
cs_compound
cs_assault
cs_office
de_mentalshutdown
cs_quebec

But they aren't dark enough. They don't give off that "dark zombie enviornment," if you understand.

I'm trying to find out how to just change some of the textures in maps and make things darker. (I have flashlight enabled on my server)

If anyone could suggest and editor with a full tutorial that I can understand, that would be great.

Thanks in advance.