Change default preferred-dark-theme to `navy`.

book-example/src/format/config.md

@@ -172,7 +172,7 @@ The following configuration options are available:
 - **preferred-dark-theme:** The default dark theme. This theme will be used if
   the browser requests the dark version of the site via the
   ['prefers-color-scheme'](https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme)
-  CSS media query. Defaults to the same theme as `default-theme`.
+  CSS media query. 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

book-example/src/format/theme/README.md

@@ -34,3 +34,9 @@ functionality. Therefore I recommend to use the file from the default theme as
 template and only add / modify what you need. You can copy the default theme
 into your source directory automatically by using `mdbook init --theme` just
 remove the files you don't want to override.
+
+If you completely replace all built-in themes, be sure to also set
+[`output.html.preferred-dark-theme`] in the config, which defaults to the
+built-in `navy` theme.
+
+[`output.html.preferred-dark-theme`]: ../config.md#html-renderer-options

src/config.rs

@@ -479,7 +479,7 @@ pub struct HtmlConfig {
     /// The default theme to use, defaults to 'light'
     pub default_theme: Option<String>,
     /// The theme to use if the browser requests the dark version of the site.
-    /// Defaults to the same as 'default_theme'
+    /// Defaults to 'navy'.
     pub preferred_dark_theme: Option<String>,
     /// Use "smart quotes" instead of the usual `"` character.
     pub curly_quotes: bool,

src/renderer/html_handlebars/hbs_renderer.rs

@@ -511,7 +511,7 @@ fn make_data(
 
     let preferred_dark_theme = match html_config.preferred_dark_theme {
         Some(ref theme) => theme.to_lowercase(),
-        None => default_theme,
+        None => "navy".to_string(),
     };
     data.insert(
         "preferred_dark_theme".to_owned(),