diff --git a/toolkit/components/extensions/ext-theme.js b/toolkit/components/extensions/ext-theme.js
index 28380e73d6e3..3c1262efb0b9 100644
--- a/toolkit/components/extensions/ext-theme.js
+++ b/toolkit/components/extensions/ext-theme.js
@@ -4,6 +4,8 @@ Cu.import("resource://gre/modules/Services.jsm");
 
 XPCOMUtils.defineLazyModuleGetter(this, "Preferences",
                                   "resource://gre/modules/Preferences.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "LightweightThemeManager",
+                                  "resource://gre/modules/LightweightThemeManager.jsm");
 
 // WeakMap[Extension -> Theme]
 let themeMap = new WeakMap();
@@ -50,6 +52,7 @@ class Theme {
     if (this.lwtStyles.headerURL &&
         this.lwtStyles.accentcolor &&
         this.lwtStyles.textcolor) {
+      LightweightThemeManager.fallbackThemeData = this.lwtStyles;
       Services.obs.notifyObservers(null,
         "lightweight-theme-styling-update",
         JSON.stringify(this.lwtStyles));
@@ -142,6 +145,7 @@ class Theme {
     for (let icon of ICONS) {
       lwtStyles.icons[`--${icon}--icon`] = "";
     }
+    LightweightThemeManager.fallbackThemeData = null;
     Services.obs.notifyObservers(null,
       "lightweight-theme-styling-update",
       JSON.stringify(lwtStyles));
diff --git a/toolkit/mozapps/extensions/LightweightThemeManager.jsm b/toolkit/mozapps/extensions/LightweightThemeManager.jsm
index 265f2bc195f5..e3e856616c2e 100644
--- a/toolkit/mozapps/extensions/LightweightThemeManager.jsm
+++ b/toolkit/mozapps/extensions/LightweightThemeManager.jsm
@@ -71,6 +71,10 @@ Object.defineProperty(this, "_maxUsedThemes", {
 var _themeIDBeingEnabled = null;
 var _themeIDBeingDisabled = null;
 
+// 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;
+
 // Convert from the old storage format (in which the order of usedThemes
 // was combined with isThemeSelected to determine which theme was selected)
 // to the new one (where a selectedThemeID determines which theme is selected).
@@ -96,6 +100,19 @@ this.LightweightThemeManager = {
     return "LightweightThemeManager";
   },
 
+  set fallbackThemeData(data) {
+    if (data && Object.getOwnPropertyNames(data).length) {
+      _fallbackThemeData = Object.assign({}, data);
+      if (PERSIST_ENABLED) {
+        LightweightThemeImageOptimizer.purge();
+        _persistImages(_fallbackThemeData, () => {});
+      }
+    } else {
+      _fallbackThemeData = null;
+    }
+    return _fallbackThemeData;
+  },
+
   // Themes that can be added for an application.  They can't be removed, and
   // will always show up at the top of the list.
   _builtInThemes: new Map(),
@@ -123,6 +140,8 @@ this.LightweightThemeManager = {
 
   get currentThemeForDisplay() {
     var data = this.currentTheme;
+    if (!data && _fallbackThemeData)
+      data = _fallbackThemeData;
 
     if (data && PERSIST_ENABLED) {
       for (let key in PERSIST_FILES) {
diff --git a/toolkit/mozapps/extensions/internal/XPIProvider.jsm b/toolkit/mozapps/extensions/internal/XPIProvider.jsm
index fcee1f3dce6b..aedd6c0293a4 100644
--- a/toolkit/mozapps/extensions/internal/XPIProvider.jsm
+++ b/toolkit/mozapps/extensions/internal/XPIProvider.jsm
@@ -7543,7 +7543,7 @@ AddonWrapper.prototype = {
     }
 
     if (addon.inDatabase) {
-      let theme = isTheme(addon.type)
+      let theme = isTheme(addon.type);
       if (theme && val) {
         if (addon.internalName == XPIProvider.defaultSkin)
           throw new Error("Cannot disable the default theme");