--- a/browser/components/customizableui/CustomizeMode.jsm
+++ b/browser/components/customizableui/CustomizeMode.jsm
@@ -148,20 +148,18 @@ CustomizeMode.prototype = {
       this.enter();
     }
   },
 
   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 + ")";
+   lwthemeIcon.style.backgroundImage = LightweightThemeManager.currentTheme ?
+     "url(" + LightweightThemeManager.currentTheme.iconURL + ")" : "";
   },
 
   setTab: function(aTab) {
     if (gTab == aTab) {
       return;
     }
 
     if (gTab) {

--- a/browser/themes/shared/customizableui/customizeMode.inc.css
+++ b/browser/themes/shared/customizableui/customizeMode.inc.css
@@ -189,16 +189,17 @@
   /* Sadly, button.css thinks its margins are perfect for everyone. */
   margin-inline-start: 6px !important;
 }
 
 #customization-lwtheme-button > .box-inherit > .box-inherit > .button-icon {
   width: 20px;
   height: 20px;
   border-radius: 2px;
+  background-image: url("chrome://browser/skin/theme-switcher-icon.png");
   background-size: contain;
 }
 
 %ifdef CAN_DRAW_IN_TITLEBAR
 #customization-titlebar-visibility-button {
   list-style-image: url("chrome://browser/skin/customizableui/customize-titleBar-toggle.png");
   -moz-image-region: rect(0, 24px, 24px, 0);
 }