Well, after peeking into the code, it seems that fog values are ship type-dependant. Except that, it seems pretty straightforward to change the values used - everything has near and far fog values, and the nebula clouds are based on two variables, which are the outer radius, behind which the cloud is transparent, and the inner radius. The nebula cloud grows less transparent until it reaches the inner radius, and from the inner radius, it grows less transparent until it reaches a "magic number" (I'm not kidding you, that's how it's written in the code)