diff --git a/crates/mdbook-core/src/config.rs b/crates/mdbook-core/src/config.rs
index b2c6862c30..e0bef5efba 100644
--- a/crates/mdbook-core/src/config.rs
+++ b/crates/mdbook-core/src/config.rs
@@ -450,10 +450,10 @@ pub struct HtmlConfig {
     /// The theme directory, if specified.
     pub theme: Option<PathBuf>,
     /// The default theme to use, defaults to 'light'
-    pub default_theme: Option<String>,
+    pub default_theme: Option<BuiltinTheme>,
     /// The theme to use if the browser requests the dark version of the site.
     /// Defaults to 'navy'.
-    pub preferred_dark_theme: Option<String>,
+    pub preferred_dark_theme: Option<BuiltinTheme>,
     /// Supports smart quotes, apostrophes, ellipsis, en-dash, and em-dash.
     pub smart_punctuation: bool,
     /// Support for definition lists.
@@ -728,6 +728,40 @@ trait Updateable<'de>: Serialize + Deserialize<'de> {
 
 impl<'de, T> Updateable<'de> for T where T: Serialize + Deserialize<'de> {}
 
+/// A built-in HTML theme supported by mdBook.
+///
+/// These themes are defined in `crates/mdbook-html/front-end/css/variables.css`.
+///
+/// The enum variants correspond to the theme names accepted by the
+/// `[output.html]` configuration keys `default-theme` and
+/// `preferred-dark-theme`.
+#[derive(Debug, Copy, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(rename_all = "lowercase", deny_unknown_fields)]
+#[non_exhaustive]
+pub enum BuiltinTheme {
+    /// A dark theme with a balanced color palette.
+    Ayu,
+    /// A dark theme with a gray-on-gray aesthetic.
+    Coal,
+    /// A light theme optimized for readability.
+    Light,
+    /// A dark navy-themed variant.
+    Navy,
+    /// A rust-colored theme.
+    Rust,
+}
+
+impl BuiltinTheme {
+    /// An array of all built-in themes, in the order they should be displayed in the theme selector.
+    pub const ALL: [BuiltinTheme; 5] = [
+        BuiltinTheme::Light,
+        BuiltinTheme::Rust,
+        BuiltinTheme::Coal,
+        BuiltinTheme::Navy,
+        BuiltinTheme::Ayu,
+    ];
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -795,7 +829,7 @@ mod tests {
             smart_punctuation: true,
             additional_css: vec![PathBuf::from("./foo/bar/baz.css")],
             theme: Some(PathBuf::from("./themedir")),
-            default_theme: Some(String::from("rust")),
+            default_theme: Some(BuiltinTheme::Rust),
             playground: playground_should_be,
             git_repository_url: Some(String::from("https://foo.com/")),
             git_repository_icon: Some(String::from("fa-code-fork")),
@@ -819,6 +853,53 @@ mod tests {
         assert_eq!(got.html_config().unwrap(), html_should_be);
     }
 
+    #[test]
+    fn parse_builtin_html_themes() {
+        let src = r#"
+        [output.html]
+        default-theme = "rust"
+        preferred-dark-theme = "coal"
+        "#;
+
+        let got = Config::from_str(src).unwrap();
+        let html_config = got.html_config().unwrap();
+
+        assert_eq!(html_config.default_theme, Some(BuiltinTheme::Rust));
+        assert_eq!(html_config.preferred_dark_theme, Some(BuiltinTheme::Coal));
+        assert_eq!(
+            BuiltinTheme::ALL,
+            [
+                BuiltinTheme::Light,
+                BuiltinTheme::Rust,
+                BuiltinTheme::Coal,
+                BuiltinTheme::Navy,
+                BuiltinTheme::Ayu,
+            ]
+        );
+    }
+
+    #[test]
+    fn invalid_default_theme() {
+        let src = r#"
+        [output.html]
+        default-theme = "does-not-exist"
+        "#;
+
+        let got = Config::from_str(src).unwrap();
+        assert_eq!(got.html_config(), None);
+    }
+
+    #[test]
+    fn invalid_preferred_dark_theme() {
+        let src = r#"
+        [output.html]
+        preferred-dark-theme = "does-not-exist"
+        "#;
+
+        let got = Config::from_str(src).unwrap();
+        assert_eq!(got.html_config(), None);
+    }
+
     #[test]
     fn disable_runnable() {
         let src = r#"
diff --git a/crates/mdbook-html/front-end/css/chrome.css b/crates/mdbook-html/front-end/css/chrome.css
index 232a8c2e85..8be5ac783b 100644
--- a/crates/mdbook-html/front-end/css/chrome.css
+++ b/crates/mdbook-html/front-end/css/chrome.css
@@ -709,6 +709,7 @@ html:not(.js) .sidebar-resize-handle {
     color: inherit;
     background: inherit;
     font-size: inherit;
+    text-transform: capitalize;
 }
 .theme-popup .theme:hover {
     background-color: var(--theme-hover);
diff --git a/crates/mdbook-html/front-end/templates/index.hbs b/crates/mdbook-html/front-end/templates/index.hbs
index b1834189f9..6a3f5d7832 100644
--- a/crates/mdbook-html/front-end/templates/index.hbs
+++ b/crates/mdbook-html/front-end/templates/index.hbs
@@ -151,11 +151,9 @@
                         </button>
                         <ul id="mdbook-theme-list" class="theme-popup" aria-label="Themes" role="menu">
                             <li role="none"><button role="menuitem" class="theme" id="mdbook-theme-default_theme">Auto</button></li>
-                            <li role="none"><button role="menuitem" class="theme" id="mdbook-theme-light">Light</button></li>
-                            <li role="none"><button role="menuitem" class="theme" id="mdbook-theme-rust">Rust</button></li>
-                            <li role="none"><button role="menuitem" class="theme" id="mdbook-theme-coal">Coal</button></li>
-                            <li role="none"><button role="menuitem" class="theme" id="mdbook-theme-navy">Navy</button></li>
-                            <li role="none"><button role="menuitem" class="theme" id="mdbook-theme-ayu">Ayu</button></li>
+                            {{#each builtin_themes}}
+                            <li role="none"><button role="menuitem" class="theme" id="mdbook-theme-{{this}}">{{this}}</button></li>
+                            {{/each}}
                         </ul>
                         {{#if search_enabled}}
                         <button id="mdbook-search-toggle" class="icon-button" type="button" title="Search (`/`)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="/ s" aria-controls="mdbook-searchbar">
diff --git a/crates/mdbook-html/src/html_handlebars/hbs_renderer.rs b/crates/mdbook-html/src/html_handlebars/hbs_renderer.rs
index 8edac3cace..15c31501a7 100644
--- a/crates/mdbook-html/src/html_handlebars/hbs_renderer.rs
+++ b/crates/mdbook-html/src/html_handlebars/hbs_renderer.rs
@@ -7,7 +7,7 @@ use crate::utils::ToUrlPath;
 use anyhow::{Context, Result, bail};
 use handlebars::Handlebars;
 use mdbook_core::book::{Book, BookItem, Chapter};
-use mdbook_core::config::{BookConfig, Config, HtmlConfig};
+use mdbook_core::config::{BookConfig, BuiltinTheme, Config, HtmlConfig};
 use mdbook_core::utils::fs;
 use mdbook_renderer::{RenderContext, Renderer};
 use serde_json::json;
@@ -491,16 +491,14 @@ fn make_data(
         );
     }
 
-    let default_theme = match html_config.default_theme {
-        Some(ref theme) => theme.to_lowercase(),
-        None => "light".to_string(),
-    };
+    data.insert("builtin_themes".to_owned(), json!(BuiltinTheme::ALL));
+
+    let default_theme = html_config.default_theme.unwrap_or(BuiltinTheme::Light);
     data.insert("default_theme".to_owned(), json!(default_theme));
 
-    let preferred_dark_theme = match html_config.preferred_dark_theme {
-        Some(ref theme) => theme.to_lowercase(),
-        None => "navy".to_string(),
-    };
+    let preferred_dark_theme = html_config
+        .preferred_dark_theme
+        .unwrap_or(BuiltinTheme::Navy);
     data.insert(
         "preferred_dark_theme".to_owned(),
         json!(preferred_dark_theme),
diff --git a/guide/src/format/configuration/renderers.md b/guide/src/format/configuration/renderers.md
index 22dfd425fb..716e657b34 100644
--- a/guide/src/format/configuration/renderers.md
+++ b/guide/src/format/configuration/renderers.md
@@ -118,12 +118,14 @@ The following configuration options are available:
 - **theme:** mdBook comes with a default theme and all the resource files needed
   for it. But if this option is set, mdBook will selectively overwrite the theme
   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 dark theme. This theme will be used if
-  the browser requests the dark version of the site via the
+- **default-theme:** The built-in theme to select by default in the
+  'Change Theme' dropdown. Accepted values are `light`, `rust`, `coal`, `navy`,
+  and `ayu`. Defaults to `light`.
+- **preferred-dark-theme:** The built-in theme to use when 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 `navy`.
+  CSS media query. Accepted values are `light`, `rust`, `coal`, `navy`, and
+  `ayu`. Defaults to `navy`.
 - **smart-punctuation:** Converts quotes to curly quotes, `...` to `…`, `--` to en-dash, and `---` to em-dash.
   See [Smart Punctuation](../markdown.md#smart-punctuation).
   Defaults to `true`.