Dark mode docs need updating for new "selector" and "variant" strategies

[MichaelAllenWarner]

With the recent addition of the "selector" and "variant" dark-mode strategies, the dark-mode documentation needs to be updated. Perhaps it should still mention the "class" strategy that's been kept for the purpose of backward compatibility?

[reinink]

Hey thanks for the reminder to do this — I've just merged in an update to this page that documents these new strategies 👍

https://tailwindcss.com/docs/dark-mode