diff --git a/src/theme/book.js b/src/theme/book.js
index 327c09fb..9551a495 100644
--- a/src/theme/book.js
+++ b/src/theme/book.js
@@ -346,12 +346,20 @@ aria-label="Show hidden lines"></button>';
             // ignore error.
         }
         if (theme === null || theme === undefined || !themeIds.includes(theme)) {
-            return default_theme;
+            if (typeof default_dark_theme === 'undefined') {
+                // A customized index.hbs might not define this, so fall back to
+                // old behavior of determining the default on page load.
+                return default_theme;
+            }
+            return window.matchMedia("(prefers-color-scheme: dark)").matches
+                ? default_dark_theme
+                : default_light_theme;
         } else {
             return theme;
         }
     }
 
+    var previousTheme = default_theme;
     function set_theme(theme, store = true) {
         let ace_theme;
 
@@ -383,8 +391,6 @@ aria-label="Show hidden lines"></button>';
             });
         }
 
-        const previousTheme = get_theme();
-
         if (store) {
             try {
                 localStorage.setItem('mdbook-theme', theme);
@@ -395,13 +401,17 @@ aria-label="Show hidden lines"></button>';
 
         html.classList.remove(previousTheme);
         html.classList.add(theme);
+        previousTheme = theme;
         updateThemeSelected();
     }
 
-    // Set theme
-    const theme = get_theme();
+    const query = window.matchMedia("(prefers-color-scheme: dark)");
+    query.onchange = function(event) {
+      set_theme(get_theme(), false);
+    };
 
-    set_theme(theme, false);
+    // Set theme.
+    set_theme(get_theme(), false);
 
     themeToggleButton.addEventListener('click', function() {
         if (themePopup.style.display === 'block') {
diff --git a/src/theme/index.hbs b/src/theme/index.hbs
index eade5ec7..6035750e 100644
--- a/src/theme/index.hbs
+++ b/src/theme/index.hbs
@@ -53,10 +53,11 @@
         <script async src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/MathJax.js?config=TeX-AMS-MML_HTMLorMML"></script>
         {{/if}}
 
-        <!-- Provide site root to javascript -->
+        <!-- Provide site root and default themes to javascript -->
         <script>
             const path_to_root = "{{ path_to_root }}";
-            const default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "{{ preferred_dark_theme }}" : "{{ default_theme }}";
+            const default_light_theme = "{{ default_theme }}";
+            const default_dark_theme = "{{ preferred_dark_theme }}";
         </script>
         <!-- Start loading toc.js asap -->
         <script src="{{ resource "toc.js" }}"></script>
@@ -81,6 +82,7 @@
 
         <!-- Set the theme before any content is loaded, prevents flash -->
         <script>
+            const default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? default_dark_theme : default_light_theme;
             let theme;
             try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
             if (theme === null || theme === undefined) { theme = default_theme; }