Is it possible to customize the sun settings? Or do you have to use the "pre defined" sun settings per skybox? (http://wiki.modsrepository.com/index.php?title=Call_of_Duty_5:_Worldspawn_Settings)

My friend once tried changing the settings and as a result, he couldnt open his map in radiant anymore and got some errors, is this just a one time thing or is it just not pissible to tweak the sun to your liking?

Thanks,

me

You can change all the worldspawn settings but its a good idea to start with those setting and tweak them. You can also change the skybox image, flare image, and a few others.
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If your friends map wouldn't open after changing worldspawn settings he probably misspelled the skybox kvp or deleted one of the worldspawn settings. You can open the map in notepad++ and paste a working maps worldspawn.

What shoudl I change to get the best looking light, but still have it dark? Because I have changed a few things but don't see any difference yet   

I have never found the perfect setting myself but these are always the first things i change when editing my worldspawn.

"suncolor" and "sunlight"
I think suncolor should be close to the skybox overall color. Sunlight anywhere from .25 to .75 for a dark map.

"ambient" and "_color" are the shadows color and how dark/intense they are. I use a low value of "_ambient .2" for a dark map and i set the "_color" to the same as the "suncolor" then darken it by dragging the slider down to nearly black in the worldspawn color window.

I always adjust one kvp at a time every time i compile and make a note of the shadows edges and color and how the weaps reflect light when walking from worldspawn lighting to radiant lights.