--- a/toolkit/components/extensions/test/browser/browser.ini
+++ b/toolkit/components/extensions/test/browser/browser.ini
@@ -28,8 +28,9 @@ skip-if = verify
 [browser_ext_themes_toolbarbutton_icons.js]
 [browser_ext_themes_toolbarbutton_colors.js]
 [browser_ext_themes_theme_transition.js]
 [browser_ext_themes_arrowpanels.js]
 [browser_ext_themes_tab_selected.js]
 [browser_ext_themes_autocomplete_popup.js]
 [browser_ext_themes_sanitization.js]
 [browser_ext_themes_findbar.js]
+[browser_ext_themes_warnings.js]

new file mode 100644
--- /dev/null
+++ b/toolkit/components/extensions/test/browser/browser_ext_themes_warnings.js
@@ -0,0 +1,131 @@
+"use strict";
+
+// This test checks that theme warnings are properly emitted.
+
+function waitForConsole(task, message) {
+  return new Promise(async resolve => {
+    SimpleTest.monitorConsole(resolve, [{
+      message: new RegExp(message),
+    }]);
+    await task();
+    SimpleTest.endMonitorConsole();
+  });
+}
+
+add_task(async function setup() {
+  SimpleTest.waitForExplicitFinish();
+});
+
+add_task(async function test_static_theme() {
+  for (const property of ["colors", "images", "properties"]) {
+    const extension = ExtensionTestUtils.loadExtension({
+      manifest: {
+        theme: {
+          [property]: {
+            such_property: "much_wow",
+          },
+        },
+      },
+    });
+    await waitForConsole(
+      extension.startup,
+      `Unrecognized theme property found: ${property}.such_property`,
+    );
+    await extension.unload();
+  }
+});
+
+add_task(async function test_dynamic_theme() {
+  let extension = ExtensionTestUtils.loadExtension({
+    manifest: {
+      permissions: ["theme"],
+    },
+    background() {
+      browser.test.onMessage.addListener((msg, details) => {
+        if (msg === "update-theme") {
+          browser.theme.update(details).then(() => {
+            browser.test.sendMessage("theme-updated");
+          });
+        } else {
+          browser.theme.reset().then(() => {
+            browser.test.sendMessage("theme-reset");
+          });
+        }
+      });
+    },
+  });
+
+  await extension.startup();
+
+  for (const property of ["colors", "images", "properties"]) {
+    extension.sendMessage("update-theme", {
+      [property]: {
+        such_property: "much_wow",
+      },
+    });
+    await waitForConsole(
+      () => extension.awaitMessage("theme-updated"),
+      `Unrecognized theme property found: ${property}.such_property`
+    );
+  }
+
+  await extension.unload();
+});
+
+add_task(async function test_experiment() {
+  Services.prefs.setBoolPref("extensions.legacy.enabled", true);
+
+  info("Testing that experiments are handled correctly when legacy pref is enabled");
+
+  let extension = ExtensionTestUtils.loadExtension({
+    manifest: {
+      theme: {
+        properties: {
+          such_property: "much_wow",
+          unknown_property: "very_unknown",
+        },
+      },
+      theme_experiment: {
+        properties: {
+          such_property: "--such-property",
+        },
+      },
+    },
+  });
+  if (!AppConstants.MOZ_ALLOW_LEGACY_EXTENSIONS) {
+    await waitForConsole(
+      extension.startup,
+      "This extension is not allowed to run theme experiments"
+    );
+  } else {
+    await waitForConsole(
+      extension.startup,
+      "Unrecognized theme property found: properties.unknown_property"
+    );
+  }
+  await extension.unload();
+
+  info("Testing that experiments are handled correctly when legacy pref is disabled");
+
+  Services.prefs.setBoolPref("extensions.legacy.enabled", false);
+
+  extension = ExtensionTestUtils.loadExtension({
+    manifest: {
+      theme: {
+        properties: {
+          such_property: "much_wow",
+        },
+      },
+      theme_experiment: {
+        properties: {
+          such_property: "--such-property",
+        },
+      },
+    },
+  });
+  await waitForConsole(
+    extension.startup,
+    "This extension is not allowed to run theme experiments"
+  );
+  await extension.unload();
+});

--- a/toolkit/components/extensions/test/xpcshell/test_ext_manifest_themes.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_manifest_themes.js
@@ -1,31 +1,28 @@
 /* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
 /* vim: set sts=2 sw=2 et tw=80: */
 "use strict";
 
 async function test_theme_property(property) {
   let normalized = await ExtensionTestUtils.normalizeManifest({
     "theme": {
       [property]: {
-        "unrecognized_key": "unrecognized_value",
       },
     },
   }, "manifest.ThemeManifest");
 
-  let expectedWarning;
   if (property === "unrecognized_key") {
-    expectedWarning = `Error processing theme.${property}`;
+    const expectedWarning = `Error processing theme.${property}`;
+    ok(normalized.errors[0].includes(expectedWarning),
+       `The manifest warning ${JSON.stringify(normalized.errors[0])} must contain ${JSON.stringify(expectedWarning)}`);
   } else {
-    expectedWarning = `Error processing theme.${property}.unrecognized_key`;
+    equal(normalized.errors.length, 0, "Should have a warning");
   }
   equal(normalized.error, undefined, "Should not have an error");
-  equal(normalized.errors.length, 1, "Should have a warning");
-  ok(normalized.errors[0].includes(expectedWarning),
-     `The manifest warning ${JSON.stringify(normalized.errors[0])} must contain ${JSON.stringify(expectedWarning)}`);
 }
 
 add_task(async function test_manifest_themes() {
   await test_theme_property("images");
   await test_theme_property("colors");
   await test_theme_property("icons");
   await test_theme_property("unrecognized_key");
 });