Illumination is documented on page 67 of the Savage Worlds Deluxe Edition rulebook and page 76 of the Savage Worlds Deluxe Explorers Edition. It states:

• Dim: Twilight, light fog, night with a full moon, and so on subtract 1 from the combatants’ attack rolls.
• Dark: Normal darkness with some ambient light (starlight, partial moon), inflicts a -2 penalty, and targets aren’t visible outside of 10″.
• Pitch Darkness: Targets are at visible at all in pitch blackness, but if a character knows roughly where a victim is, he may be attacked at -4.