Support mdbook-quiz addon

[uncenter]

Support https://github.com/cognitive-engineering-lab/mdbook-quiz. Should be as simple as:

.mdbook-quiz .answer .answer-row > div {
  .correct::before {
    color: @green;
  }
  .incorrect:before {
    color: @red;
  }
}

Though not sure if it should go in the main file or separately like it is for the admonish plugin.

[sgoudham]

I think it should go in a separate file so there's nicer separation between the default mdBook tokens and the ecosystem plugins.

If people want to, they can merge the files themselves since it is all just CSS at the end of the day.