Attack Modifiers

Cover	Attackers suffer a penalty when attempting to hit a target behind cover
Light Cover	-1	Characters subtract 1 from their attack rolls if half or less of their target is obscured.
Medium Cover	-2	The penalty is increased to -2 if more than half of the target is hidden from view. This is the usual penalty for attacking a prone character (see Prone).
Heavy Cover	-4	The penalty is -4 if only a small part of the target is visible (prone beside a tree, behind a high wall, peeking around the corner of a building, etc.).
Near Total Cover	-6	Attacking through a very tight opening that provides near total cover, such as an arrow slit, subtracts 6 from enemy attack rolls.
Illumination	Attacking a poorly-illuminated target is more difficult than a similar attack with good lighting, and incurs the following penalties
Dim	-1	Twilight, light fog, night with a full moon, and so on subtract 1 from combatants' attack rolls.
Dark	-2	Normal darkness with some ambient light (starlight, partial moon) inflicts a -2 penalty, and targets aren't visible outside of 10“.
Pitch Darkness:	-4	Targets aren't visible at all in pitch blackness, but if a character knows roughly where a victim is (he can hear him, target is in a confined space, a glint of light shines off his blade, etc.), he may be attacked at -4.

Savage Worlds Deluxe Edition p. 76

Savage Worlds Deluxe