simple.diff

diff --git a/browser/components/customizableui/CustomizeMode.jsm b/browser/components/customizableui/CustomizeMode.jsm
--- a/browser/components/customizableui/CustomizeMode.jsm
+++ b/browser/components/customizableui/CustomizeMode.jsm
@@ -119,19 +119,20 @@ CustomizeMode.prototype = {
     }
   },
 
   swatchForTheme: function(aDocument) {
    let lwthemeButton = aDocument.getElementById("customization-lwtheme-button");
    let lwthemeIcon = aDocument.getAnonymousElementByAttribute(lwthemeButton,
           "class", "button-icon");
    let imageURL = LightweightThemeManager.currentTheme === null ?
-          "chrome://browser/skin/theme-switcher-icon.png" :
-          LightweightThemeManager.currentTheme.iconURL;
-    lwthemeIcon.style.backgroundImage = "url(" + imageURL + ")";
+          "url(chrome://browser/skin/theme-switcher-icon.png)" :
+          "url(" + LightweightThemeManager.currentTheme.iconURL + "), " +
+          "url(chrome://browser/skin/theme-switcher-icon.png)";
+    lwthemeIcon.style.backgroundImage = imageURL;
   },
 
   enter: function() {
     this._wantToBeInCustomizeMode = true;
 
     if (this._customizing || this._handler.isEnteringCustomizeMode) {
       return;
     }