Dimly comes from the adjective dim, whose root is the Old English dimm, "dark or obscure," from Germanic origins.

Is dimly a verb or noun?

dimly adverb - Definition, pictures, pronunciation and usage notes | Oxford Advanced Learner's Dictionary at OxfordLearnersDictionaries.com.

Is brightness a noun or adjective?

noun. /ˈbraɪtnəs/ /ˈbraɪtnəs/ [uncountable] ​the fact of being full of light or of shining strongly.