docs: Improve dark mode syntax highlighting (#18002)

This PR introduces [GitHub
Light](https://github.com/highlightjs/highlight.js/blob/main/src/styles/github.css)
and [GitHub
Dark](https://github.com/highlightjs/highlight.js/blob/main/src/styles/github-dark.css)
as the syntax highlighting themes for the corresponding modes.

Release Notes:

- N/A

docs/book.toml

@@ -8,8 +8,8 @@ site-url = "/docs/"
 
 [output.html]
 no-section-label = true
-preferred-dark-theme = "light"
-additional-css = ["theme/page-toc.css", "theme/plugins.css"]
+preferred-dark-theme = "dark"
+additional-css = ["theme/page-toc.css", "theme/plugins.css", "theme/highlight.css"]
 additional-js  = ["theme/page-toc.js", "theme/plugins.js"]
 
 [output.html.print]