Document preferred-dark-theme config option

book-example/src/format/config.md

@@ -150,6 +150,8 @@ The following configuration options are available:
   files with the ones found in the specified folder.
 - **default-theme:** The theme color scheme to select by default in the
   'Change Theme' dropdown. Defaults to `light`.
+- **preferred-dark-theme:** The default theme to use if the browser
+  requests the dark version of the site. Defaults to `navy`.
 - **curly-quotes:** Convert straight quotes to curly quotes, except for those
   that occur in code blocks and code spans. Defaults to `false`.
 - **mathjax-support:** Adds support for [MathJax](mathjax.md). Defaults to