Nibbles, 15 1414 13 bytes
=%+=+.@o$2%`D@o$%`D-21~8fd905cff4e
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Explanation
@ - input
. o$ - as codepoints
+ - sum
% 2 - modulo 2
= %`D-21~8fd905cff4e - index in compressed list ["light", "dark"] (wrapped)
-1 byte2 bytes thanks to Dominic van Essen