add an id to the core-item inside the shadow-root so that it can be

targeted for styling

core-submenu.html

@@ -32,6 +32,12 @@
       margin-left: 20px;
     }
 
+To style the item for the submenu, do something like this:
+
+    core-submenu::shadow > #submenuItem {
+      color: blue;
+    }
+
 @group Polymer Core Elements
 @element core-submenu
 @extends core-item
@@ -46,7 +52,7 @@
 
   <link rel="stylesheet" href="core-submenu.css">
 
-  <core-item src="{{src}}" label="{{label}}" icon="{{icon}}" class="{{ {'core-selected' : active} | tokenList}}" on-tap="{{activate}}">
+  <core-item id="submenuItem" src="{{src}}" label="{{label}}" icon="{{icon}}" class="{{ {'core-selected' : active} | tokenList}}" on-tap="{{activate}}">
     <content select=".item-content"></content>
   </core-item>