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diff --git a/toolkit/mozapps/extensions/LightweightThemeManager.sys.mjs b/toolkit/mozapps/extensions/LightweightThemeManager.sys.mjs
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+++ b/toolkit/mozapps/extensions/LightweightThemeManager.sys.mjs
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// Holds optional fallback theme data that will be returned when no data for an
+// active theme can be found. This the case for WebExtension Themes, for example.
+var _fallbackThemeData = null;
+
+export var LightweightThemeManager = {
+  set fallbackThemeData(data) {
+    if (data && Object.getOwnPropertyNames(data).length) {
+      _fallbackThemeData = Object.assign({}, data);
+    } else {
+      _fallbackThemeData = null;
+    }
+  },
+
+  /*
+   * Returns the currently active theme, taking the fallback theme into account
+   * if we'd be using the default theme otherwise.
+   *
+   * This will always return the original theme data and not make use of
+   * locally persisted resources.
+   */
+  get currentThemeWithFallback() {
+    return _fallbackThemeData && _fallbackThemeData.theme;
+  },
+
+  get themeData() {
+    return _fallbackThemeData || { theme: null };
+  },
+};