make `prefers-color-scheme` work in package `docs`

[peterbecich]

Continuation of https://github.com/haskell/hackage-server/issues/1007

prefers-color-scheme works here: https://hackage.haskell.org/package/timeline-0.1.1.0 It does not work here: https://hackage.haskell.org/package/timeline-0.1.1.0/docs/Data-Timeline-Hedgehog.html

I did not cover this here https://github.com/haskell/hackage-server/pull/1008 because it was not clear how docs are generated.

Would all old docs need to be regenerated to support prefers-color-scheme? Noted docs do not automatically get the latest CSS styling, i.e.: https://hackage.haskell.org/package/ABList-0.0.3/docs/Data-AbneList.html

More than likely this prefers-color-scheme support would only apply for new docs in the future.