PlatyPar, 21 bytes
This is a non-competing answer, as I made this language after the posting of this question. It was neither made for this question nor has it been augmented to fit this specific question.
X,u#^2%?"dark"\"light
Explanation
X,u#^ ## charcode of the letter XOR the number
2%? \ ## if it is odd
"dark" ## output "dark"
"light ## else output "light