Suggestion: use `:not(.is-dark)` as lightSelector by default

[aletorrado]

Hi,

This is a very subjective matter, but I wanted to suggest you for the sake of simplicity, to just use a negated :not(.is-dark) selector as the default lightSelector. Even, the plugin may compute something like :not(<darkSelector>), so that the user can safely override darkSelector preserving the same behavior.

Using a single class or attribute seems like a good idea to simplify JS manipulation and avoid invalid states if by any means both is-light and is-dark are triggered.

If you agree with this, I would be glad to submit a PR.

Thanks!