Whether a word is an adverb or not, and whether is is used as an adverb, are two different things.
While most adverbs end in -ly, they don't have to. At least some (if not most) grammarians consider anything that modifies an adjective an adverb, meaning that indeed, light and dark are adverbs in your sentence.
This source puts it as
adjective use as an adverb
Some of their examples:
The woman is quite pretty.
This book is more interesting than the last one.
The weather report is almost always right.
The adverb almost is modifying the adverb always, and they’re both modifying right.