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

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.