Thunno B
, \$18 \log_{256}(96) \approx\$\$17 \log_{256}(96) \approx\$ 14.82 bytes
S"lightdark"5ApsAHS"ldiagrhkt"ZlsAH
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S"lightdark"5ApsAHS"ldiagrhkt"ZlsAH # Implicit input. The B flag gets the ordinals of each character in the input.
S # Sum the ordinals
"lightdark" "ldiagrhkt" # String "lightdark""ldiagrhkt"
5ApZl # SplitUninterleaved intoto get ['light', 'dark']
sAH # Index into this list (1-based, modular)
# Implicit output