replace uses of #browser-bottombox with #main-window > body::after

Browser-bottombox element has been removed from Firefox in 109 so lets
not use that anymore.

content/standalone_image_page_mods.css

@@ -3,7 +3,6 @@ See the above repository for updates as well as full license text. */
 
 @-moz-document media-document(image){
   @media not print {
-    
     :root{
       background-color: #232325 !important; /* set solid color background */
       background-image: none !important; /* remove default background */