Is there any clarification on when/how light levels apply to mass combat or is it all GM discretion?
For example:
Normal Army camped at night with a reasonable amount of campfires vs attacking Darkvision Army - Is this now considered a dim light battle? or is it not enough light and still counts as dark?
In some mass combats, the specifics of a battlefield won't impact either army, but sometimes the battlefield will itself decide the outcome. The modifiers listed below apply only for the duration of the battle. Naturally, the GM should exercise judgment regarding any conditions that don't seem to apply to one of the armies (such as darkness and an army with darkvision, or fog and an army with scent).
Seems like the rules intentionally leave such things up to the DM, for reasons of simplicity.
Mass combat is complex enough without designing mechanics to cover such things. Too many variables. Make a call that seems reasonable in the situation at hand.