APL (Dyalog Unicode), 2525 24 bytes^SBCS

-1 thanks to Bubbler.

'dark'⊃'dark' 'light'⊃⍨2|+'light'⌽⍨+/⎕UCS⍞

Try it online!

⍞ get text from stdin

⎕UCS convert to Universal Character Set code points

+/ sum

'dark' 'light'⌽⍨ use that to cyclically rotate the list of strings

⊃ pick the first

APL (Dyalog Unicode), 25 bytes^SBCS

Requires ⎕IO←0 (zero-indexing)1 thanks to Bubbler.

'dark' 'light'⊃⍨2|+/⎕UCS⍞

Try it online!Try it online!

⍞ get text from stdin

⎕UCS convert to Universal Character Set code points

+/ sum

2| 2-mod

'dark' 'light'⊃⍨'light'⌽⍨ use that to pick fromcyclically rotate the list of strings

⊃ pick the first