Fix: don't assign undefined to style object (#764)

js/usercss.js

@@ -43,8 +43,10 @@ const usercss = (() => {
         style.usercssData = metadata;
         // https://github.com/openstyles/stylus/issues/560#issuecomment-440561196
         for (const [key, value] of Object.entries(GLOBAL_METAS)) {
+          if (metadata[key] !== undefined) {
             style[value || key] = metadata[key];
           }
+        }
         return style;
       });
   }