Feature: dark mode docs

[ilike314272]

Added dark mode by editing mkdocs.yml at request of #4913.

[vitaut]

Thanks for the PR.

Could you make the syntax highlighting follow the selected color scheme as well? We currently load Highlight.js's default.min.css unconditionally, which is a light theme, so code blocks will remain light in dark mode.

It would be good to add dark-mode highlighting styles scoped to [data-md-color-scheme="slate"] (and keep the current styles for the default scheme).

[ilike314272]

New dark.css commit adds syntax highlighting that looks more pleasing than copying light mode css

[vitaut]

Thanks. Could we avoid adding a separate dark.css? The current file is essentially a copy of Highlight.js's Atom One Dark theme, which also brings in third-party licensing concerns.

I think a cleaner approach would be to keep the palette change in mkdocs.yml and add a small set of Highlight.js overrides to fmt.css that use MkDocs Material's existing code color variables. That way the highlighting follows the selected light/dark scheme without maintaining a separate hard-coded theme.

[ilike314272]

I noticed that using material theme would have kept the banner from being blue like the Ukraine flag so I also provided override for theme on banner. dark.css removed and replaced it with .hjls overrides at bottom of fmt.css. Thank you for reviewing, I am learning from it.