diff --git a/src/librustdoc/config.rs b/src/librustdoc/config.rs
index 56b40d8c66baf..2c514a0c8267b 100644
--- a/src/librustdoc/config.rs
+++ b/src/librustdoc/config.rs
@@ -509,7 +509,7 @@ impl Options {
                 // these values up both in `dataset` and in the storage API, so it needs to be able
                 // to convert the names back and forth.  Despite doing this kebab-case to
                 // StudlyCaps transformation automatically, the JS DOM API does not provide a
-                // mechanism for doing the just transformation on a string.  So we want to avoid
+                // mechanism for doing just the transformation on a string.  So we want to avoid
                 // the StudlyCaps representation in the `dataset` property.
                 //
                 // We solve this by replacing all the `-`s with `_`s.  We do that here, when we
diff --git a/src/librustdoc/html/static/js/settings.js b/src/librustdoc/html/static/js/settings.js
index a841b4b63bae8..1cd552e7f25b7 100644
--- a/src/librustdoc/html/static/js/settings.js
+++ b/src/librustdoc/html/static/js/settings.js
@@ -1,5 +1,5 @@
 // Local js definitions:
-/* global getSettingValue, getVirtualKey, updateLocalStorage, updateSystemTheme */
+/* global getSettingValue, getVirtualKey, updateLocalStorage, updateTheme */
 /* global addClass, removeClass, onEach, onEachLazy, blurHandler, elemIsInParent */
 /* global MAIN_ID, getVar, getSettingsButton */
 
@@ -19,7 +19,7 @@
             case "theme":
             case "preferred-dark-theme":
             case "preferred-light-theme":
-                updateSystemTheme();
+                updateTheme();
                 updateLightAndDark();
                 break;
             case "line-numbers":
diff --git a/src/librustdoc/html/static/js/storage.js b/src/librustdoc/html/static/js/storage.js
index db2db83ca6310..8836d1b2e464b 100644
--- a/src/librustdoc/html/static/js/storage.js
+++ b/src/librustdoc/html/static/js/storage.js
@@ -153,79 +153,74 @@ function switchTheme(styleElem, mainStyleElem, newThemeName, saveTheme) {
     }
 }
 
-// This function is called from "main.js".
-// eslint-disable-next-line no-unused-vars
-function useSystemTheme(value) {
-    if (value === undefined) {
-        value = true;
-    }
-
-    updateLocalStorage("use-system-theme", value);
-
-    // update the toggle if we're on the settings page
-    const toggle = document.getElementById("use-system-theme");
-    if (toggle && toggle instanceof HTMLInputElement) {
-        toggle.checked = value;
-    }
-}
-
-const updateSystemTheme = (function() {
-    if (!window.matchMedia) {
-        // fallback to the CSS computed value
-        return () => {
-            const cssTheme = getComputedStyle(document.documentElement)
-                .getPropertyValue("content");
-
-            switchTheme(
-                window.currentTheme,
-                window.mainTheme,
-                JSON.parse(cssTheme) || "light",
-                true
-            );
+const updateTheme = (function() {
+    /**
+     * Update the current theme to match whatever the current combination of
+     * * the preference for using the system theme
+     *   (if this is the case, the value of preferred-light-theme, if the
+     *   system theme is light, otherwise if dark, the value of
+     *   preferred-dark-theme.)
+     * * the preferred theme
+     * … dictates that it should be.
+     */
+    function updateTheme() {
+        const use = (theme, saveTheme) => {
+            switchTheme(window.currentTheme, window.mainTheme, theme, saveTheme);
         };
-    }
-
-    // only listen to (prefers-color-scheme: dark) because light is the default
-    const mql = window.matchMedia("(prefers-color-scheme: dark)");
 
-    function handlePreferenceChange(mql) {
-        const use = theme => {
-            switchTheme(window.currentTheme, window.mainTheme, theme, true);
-        };
         // maybe the user has disabled the setting in the meantime!
         if (getSettingValue("use-system-theme") !== "false") {
             const lightTheme = getSettingValue("preferred-light-theme") || "light";
             const darkTheme = getSettingValue("preferred-dark-theme") || "dark";
 
-            if (mql.matches) {
-                use(darkTheme);
+            if (isDarkMode()) {
+                use(darkTheme, true);
             } else {
                 // prefers a light theme, or has no preference
-                use(lightTheme);
+                use(lightTheme, true);
             }
             // note: we save the theme so that it doesn't suddenly change when
             // the user disables "use-system-theme" and reloads the page or
             // navigates to another page
         } else {
-            use(getSettingValue("theme"));
+            use(getSettingValue("theme"), false);
         }
     }
 
-    mql.addListener(handlePreferenceChange);
+    // This is always updated below to a function () => bool.
+    let isDarkMode;
 
-    return () => {
-        handlePreferenceChange(mql);
-    };
-})();
+    // Determine the function for isDarkMode, and if we have
+    // `window.matchMedia`, set up an event listener on the preferred color
+    // scheme.
+    //
+    // Otherwise, fall back to the prefers-color-scheme value CSS captured in
+    // the "content" property.
+    if (window.matchMedia) {
+        // only listen to (prefers-color-scheme: dark) because light is the default
+        const mql = window.matchMedia("(prefers-color-scheme: dark)");
 
-function switchToSavedTheme() {
-    switchTheme(
-        window.currentTheme,
-        window.mainTheme,
-        getSettingValue("theme") || "light",
-        false
-    );
-}
+        isDarkMode = () => mql.matches;
+
+        if (mql.addEventListener) {
+            mql.addEventListener("change", updateTheme);
+        } else {
+            // This is deprecated, see:
+            // https://developer.mozilla.org/en-US/docs/Web/API/MediaQueryList/addListener
+            mql.addListener(updateTheme);
+        }
+    } else {
+        // fallback to the CSS computed value
+        const cssContent = getComputedStyle(document.documentElement)
+            .getPropertyValue("content");
+        // (Note: the double-quotes come from that this is a CSS value, which
+        // might be a length, string, etc.)
+        const cssColorScheme = cssContent || "\"light\"";
+        isDarkMode = () => (cssColorScheme === "\"dark\"");
+    }
+
+    return updateTheme;
+})();
 
 if (getSettingValue("use-system-theme") !== "false" && window.matchMedia) {
     // update the preferred dark theme if the user is already using a dark theme
@@ -235,13 +230,10 @@ if (getSettingValue("use-system-theme") !== "false" && window.matchMedia) {
         && darkThemes.indexOf(localStoredTheme) >= 0) {
         updateLocalStorage("preferred-dark-theme", localStoredTheme);
     }
-
-    // call the function to initialize the theme at least once!
-    updateSystemTheme();
-} else {
-    switchToSavedTheme();
 }
 
+updateTheme();
+
 if (getSettingValue("source-sidebar-show") === "true") {
     // At this point in page load, `document.body` is not available yet.
     // Set a class on the `<html>` element instead.
@@ -259,6 +251,6 @@ if (getSettingValue("source-sidebar-show") === "true") {
 // specifically when talking to a remote website with no caching.
 window.addEventListener("pageshow", ev => {
     if (ev.persisted) {
-        setTimeout(switchToSavedTheme, 0);
+        setTimeout(updateTheme, 0);
     }
 });