Nibbles, 15 14 bytes

=%+.@o$2%`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"]

-1 byte thanks to Dominic van Essen

Nibbles, 15 14 13 bytes

=+.@o$%`D-21~8fd905cff4e

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Explanation

   @                      - input
  . o$                    - as codepoints
 +                        - sum
=     %`D-21~8fd905cff4e  - index in compressed list ["light", "dark"] (wrapped)

-2 bytes thanks to Dominic van Essen

Nibbles, 15 bytes

=%+.@o$2"light""dark"

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Explanation

    @                    - input
   . o$                  - as codepoints
  +                      - sum
 %     2                 - modulo 2
=       "light""dark"    - index in ["light", "dark"]

Nibbles, 15 14 bytes

=%+.@o$2%`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"]

-1 byte thanks to Dominic van Essen

Nibbles, 15 bytes

=%+.@o$2"light""dark"

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Explanation

    @                    - input
   . o                   - as codepoints
  +                      - sum
 %    $2                 - modulo 2
=       "light""dark"    - index in ["light", "dark"]

Nibbles, 15 bytes

=%+.@o$2"light""dark"

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Explanation

    @                    - input
   . o$                  - as codepoints
  +                      - sum
 %     2                 - modulo 2
=       "light""dark"    - index in ["light", "dark"]