--- a/toolkit/components/extensions/parent/ext-browserSettings.js
+++ b/toolkit/components/extensions/parent/ext-browserSettings.js
@@ -171,16 +171,25 @@ ExtensionPreferencesManager.addSetting("
   permission: "browserSettings",
   prefNames: ["browser.display.document_color_use"],
 
   setCallback(value) {
     return { [this.prefNames[0]]: value };
   },
 });
 
+ExtensionPreferencesManager.addSetting("overrideContentColorScheme", {
+  permission: "browserSettings",
+  prefNames: ["layout.css.prefers-color-scheme.content-override"],
+
+  setCallback(value) {
+    return { [this.prefNames[0]]: value };
+  },
+});
+
 ExtensionPreferencesManager.addSetting("useDocumentFonts", {
   permission: "browserSettings",
   prefNames: ["browser.display.use_document_fonts"],
 
   setCallback(value) {
     return { [this.prefNames[0]]: value };
   },
 });
@@ -430,16 +439,53 @@ this.browserSettings = class extends Ext
               return ExtensionPreferencesManager.setSetting(
                 extension.id,
                 "overrideDocumentColors",
                 prefValue
               );
             },
           }
         ),
+        overrideContentColorScheme: Object.assign(
+          getSettingsAPI({
+            context,
+            name: "overrideContentColorScheme",
+            callback() {
+              let prefValue = Services.prefs.getIntPref(
+                "layout.css.prefers-color-scheme.content-override"
+              );
+              if (prefValue === 0) {
+                return "dark";
+              } else if (prefValue === 1) {
+                return "light";
+              }
+              return "system";
+            },
+          }),
+          {
+            set: details => {
+              if (!["light", "dark", "system"].includes(details.value)) {
+                throw new ExtensionError(
+                  `${details.value} is not a valid value for overrideContentColorScheme.`
+                );
+              }
+              let prefValue = 2; // initialize to 2 - system colors
+              if (details.value === "light") {
+                prefValue = 1;
+              } else if (details.value === "dark") {
+                prefValue = 0;
+              }
+              return ExtensionPreferencesManager.setSetting(
+                extension.id,
+                "overrideContentColorScheme",
+                prefValue
+              );
+            },
+          }
+        ),
         useDocumentFonts: Object.assign(
           getSettingsAPI({
             context,
             name: "useDocumentFonts",
             callback() {
               return (
                 Services.prefs.getIntPref(
                   "browser.display.use_document_fonts"

--- a/toolkit/components/extensions/schemas/browser_settings.json
+++ b/toolkit/components/extensions/schemas/browser_settings.json
@@ -96,16 +96,20 @@
       "webNotificationsDisabled": {
         "$ref": "types.Setting",
         "description": "Disables webAPI notifications."
       },
       "overrideDocumentColors": {
         "$ref": "types.Setting",
         "description": "This setting controls whether the user-chosen colors override the page's colors."
       },
+      "overrideContentColorScheme": {
+        "$ref": "types.Setting",
+        "description": "This setting controls whether a light or dark color scheme overrides the page's preferred color scheme."
+      },
       "useDocumentFonts": {
         "$ref": "types.Setting",
         "description": "This setting controls whether the document's fonts are used."
       },
       "zoomFullPage": {
         "$ref": "types.Setting",
         "description": "This boolean setting controls whether zoom is applied to the full page or to text only."
       },

--- a/toolkit/components/extensions/test/xpcshell/test_ext_browserSettings.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_browserSettings.js
@@ -43,16 +43,17 @@ add_task(async function test_browser_set
     "permissions.default.desktop-notification": PERM_UNKNOWN_ACTION,
     "ui.context_menus.after_mouseup": false,
     "browser.tabs.closeTabByDblclick": false,
     "browser.tabs.loadBookmarksInTabs": false,
     "browser.search.openintab": false,
     "browser.tabs.insertRelatedAfterCurrent": true,
     "browser.tabs.insertAfterCurrent": false,
     "browser.display.document_color_use": 1,
+    "layout.css.prefers-color-scheme.content-override": 2,
     "browser.display.use_document_fonts": 1,
     "browser.zoom.full": true,
     "browser.zoom.siteSpecific": true,
   };
 
   async function background() {
     let listeners = new Set([]);
     browser.test.onMessage.addListener(async (msg, apiName, value) => {
@@ -266,16 +267,26 @@ add_task(async function test_browser_set
   });
   await testSetting("overrideDocumentColors", "never", {
     "browser.display.document_color_use": 1,
   });
   await testSetting("overrideDocumentColors", "always", {
     "browser.display.document_color_use": 2,
   });
 
+  await testSetting("overrideContentColorScheme", "dark", {
+    "layout.css.prefers-color-scheme.content-override": 0,
+  });
+  await testSetting("overrideContentColorScheme", "light", {
+    "layout.css.prefers-color-scheme.content-override": 1,
+  });
+  await testSetting("overrideContentColorScheme", "system", {
+    "layout.css.prefers-color-scheme.content-override": 2,
+  });
+
   await testSetting("useDocumentFonts", false, {
     "browser.display.use_document_fonts": 0,
   });
   await testSetting("useDocumentFonts", true, {
     "browser.display.use_document_fonts": 1,
   });
 
   await testSetting("zoomFullPage", true, {
@@ -335,16 +346,28 @@ add_task(async function test_bad_value()
     );
 
     await browser.test.assertRejects(
       browser.browserSettings.overrideDocumentColors.set({ value: "bad" }),
       /bad is not a valid value for overrideDocumentColors/,
       "overrideDocumentColors.set rejects with an invalid value."
     );
 
+    await browser.test.assertRejects(
+      browser.browserSettings.overrideContentColorScheme.set({ value: 0 }),
+      /0 is not a valid value for overrideContentColorScheme/,
+      "overrideContentColorScheme.set rejects with an invalid value."
+    );
+
+    await browser.test.assertRejects(
+      browser.browserSettings.overrideContentColorScheme.set({ value: "bad" }),
+      /bad is not a valid value for overrideContentColorScheme/,
+      "overrideContentColorScheme.set rejects with an invalid value."
+    );
+
     browser.test.sendMessage("done");
   }
 
   let extension = ExtensionTestUtils.loadExtension({
     background,
     manifest: {
       permissions: ["browserSettings"],
     },