ٱلْحَمْدُ لِلَّهِ ٱلَّذِى خَلَقَ ٱلسَّمَٰوَٰتِ وَٱلْأَرْضَ وَجَعَلَ ٱلظُّلُمَٰتِ وَٱلنُّورَ ثُمَّ ٱلَّذِينَ كَفَرُوا۟ بِرَبِّهِمْ يَعْدِلُونَ
"Praise belongs to God, who created the heavens and the earth and made the darknesses and the light. Then those who deny set up equals to their Lord."
ٱلظُّلُمَٰتِ is plural, ٱلنُّورَ is singular. This usage was treated at al-Baqara 2/257 and it was recorded there: darkness is the absence of light, and the forms of absence are many; light is one. I rely on that treatment.
The two branches of the root are worth recording, and I give this as an observation: ʿadl means both equivalence and justice. The verse uses the "treating as equal" branch — that is, the associating of partners comes from the same place as the justice branch of the root: counting a thing equal to a place that is not its own. And it was treated at Luqmān 31/13 that associating partners is named ẓulm (putting a thing somewhere other than its place). The two roots give the same picture.