Add is-dark input and select inside the documentation

Is your feature request related to a problem? Please describe. We have nes-input.is-dark, but for some reason we don't show it inside the documentation anywhere (e.g. index.html). We should add it because we are have the other components is-dark example there. If #334 is accepted, add a nes-select.is-dark example as well.

Describe the solution you'd like

Add a is-dark input example below is-error input in index.html
Add a is-dark select example below is-error select in index.html (after #334).

Describe alternatives you've considered Not adding it or waiting until we find a proper way to show it.

Additional context Basic example of the select one:

nostalgic-css / NES.css

Add is-dark input and select inside the documentation #336