Thunno B, \$18 \log_{256}(96) \approx\$ 14.82 bytes

S"lightdark"5ApsAH

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S"lightdark"5ApsAH   # Implicit input. The B flag gets the ordinals of each character in the input.
S                    # Sum the ordinals
 "lightdark"         # String "lightdark"
            5Ap      # Split into ['light', 'dark']
               sAH   # Index into this list (1-based, modular)
                     # Implicit output