Adopt ::where selector to class variants for making styles overrideable

[yhatt]

Changed to use :where() selector for defining the built-in themes class variants such as invert. It makes variant styles overrideable through <style> inline style by the Markdown author easily.

v2

<!-- class: invert -->

<style>
section {
  /* ❗️ This override will not work unless using !important */
  background: gray;
}
</style>

Invert variant

v3

<!-- class: invert -->

<style>
section {
  background: gray;
}
</style>

Invert variant

Related: #244