--- a/toolkit/components/extensions/schemas/theme.json
+++ b/toolkit/components/extensions/schemas/theme.json
@@ -17,17 +17,18 @@
               "headerURL": {
                 "type": "string",
                 "optional": true
               },
               "theme_frame": {
                 "type": "string",
                 "optional": true
               }
-            }
+            },
+            "additionalProperties": { "$ref": "UnrecognizedProperty" }
           },
           "colors": {
             "type": "object",
             "optional": true,
             "properties": {
               "accentcolor": {
                 "type": "string",
                 "optional": true
@@ -45,17 +46,18 @@
                   "type": "number"
                 },
                 "optional": true
               },
               "textcolor": {
                 "type": "string",
                 "optional": true
               }
-            }
+            },
+            "additionalProperties": { "$ref": "UnrecognizedProperty" }
           },
           "icons": {
             "type": "object",
             "optional": true,
             "properties": {
               "back": {
                 "$ref": "ExtensionURL",
                 "optional": true
@@ -175,19 +177,21 @@
               "forget": {
                 "$ref": "ExtensionURL",
                 "optional": true
               },
               "pocket": {
                 "$ref": "ExtensionURL",
                 "optional": true
               }
-            }
+            },
+            "additionalProperties": { "$ref": "UnrecognizedProperty" }
           }
-        }
+        },
+        "additionalProperties": { "$ref": "UnrecognizedProperty" }
       },
       {
         "$extend": "WebExtensionManifest",
         "properties": {
           "theme": {
             "optional": true,
             "$ref": "ThemeType"
           }

new file mode 100644
--- /dev/null
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_manifest_themes.js
@@ -0,0 +1,31 @@
+/* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set sts=2 sw=2 et tw=80: */
+"use strict";
+
+function* test_theme_property(property) {
+  let normalized = yield ExtensionTestUtils.normalizeManifest({
+    "theme": {
+      [property]: {
+        "unrecognized_key": "unrecognized_value",
+      },
+    },
+  });
+
+  let expectedWarning;
+  if (property === "unrecognized_key") {
+    expectedWarning = `Error processing theme.${property}`;
+  } else {
+    expectedWarning = `Error processing theme.${property}.unrecognized_key`;
+  }
+  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(function* test_manifest_themes() {
+  yield test_theme_property("images");
+  yield test_theme_property("colors");
+  yield test_theme_property("icons");
+  yield test_theme_property("unrecognized_key");
+});

--- a/toolkit/components/extensions/test/xpcshell/xpcshell.ini
+++ b/toolkit/components/extensions/test/xpcshell/xpcshell.ini
@@ -44,16 +44,17 @@ skip-if = release_or_beta
 [test_ext_idle.js]
 [test_ext_json_parser.js]
 [test_ext_localStorage.js]
 [test_ext_management.js]
 [test_ext_management_uninstall_self.js]
 [test_ext_manifest_content_security_policy.js]
 [test_ext_manifest_incognito.js]
 [test_ext_manifest_minimum_chrome_version.js]
+[test_ext_manifest_themes.js]
 [test_ext_onmessage_removelistener.js]
 skip-if = true # This test no longer tests what it is meant to test.
 [test_ext_privacy.js]
 [test_ext_privacy_disable.js]
 [test_ext_privacy_update.js]
 [test_ext_runtime_connect_no_receiver.js]
 [test_ext_runtime_getBrowserInfo.js]
 [test_ext_runtime_getPlatformInfo.js]