diff --git a/browser/components/customizableui/CustomizeMode.jsm b/browser/components/customizableui/CustomizeMode.jsm
index 457a96321746..1022b76234db 100644
--- a/browser/components/customizableui/CustomizeMode.jsm
+++ b/browser/components/customizableui/CustomizeMode.jsm
@@ -153,10 +153,8 @@ CustomizeMode.prototype = {
    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 + ")";
+   lwthemeIcon.style.backgroundImage = LightweightThemeManager.currentTheme ?
+     "url(" + LightweightThemeManager.currentTheme.iconURL + ")" : "";
   },
 
   setTab: function(aTab) {
diff --git a/browser/themes/shared/customizableui/customizeMode.inc.css b/browser/themes/shared/customizableui/customizeMode.inc.css
index d83df6b7a579..48681d9f3c83 100644
--- a/browser/themes/shared/customizableui/customizeMode.inc.css
+++ b/browser/themes/shared/customizableui/customizeMode.inc.css
@@ -194,6 +194,7 @@
   width: 20px;
   height: 20px;
   border-radius: 2px;
+  background-image: url("chrome://browser/skin/theme-switcher-icon.png");
   background-size: contain;
 }