chore: Mezzanine border on sidecar demo page

Author: georgylobko

pages/app-layout/sidecar-demo.page.tsx

@@ -1,6 +1,6 @@
 // Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
-import React, { useContext, useState } from 'react';
+import React, { useContext, useEffect, useState } from 'react';
 
 import {
   AppLayout,
@@ -52,6 +52,13 @@ export default function WithDrawers() {
     setToolsOpen(true);
   }
 
+  useEffect(() => {
+    const headerEl = document.querySelector('#h');
+    if (headerEl && headerEl instanceof HTMLElement) {
+      headerEl!.style.borderBottom = '1px solid #424650';
+    }
+  }, []);
+
   return (
     <AppLayout
       ariaLabels={{ ...appLayoutLabels, ...drawerLabels }}