diff --git a/client/gatehouse-cli.py b/client/gatehouse-cli.py
index 7192475..97c4505 100755
--- a/client/gatehouse-cli.py
+++ b/client/gatehouse-cli.py
@@ -49,10 +49,96 @@ class MyServer(BaseHTTPRequestHandler):
         self.send_response(200)
         self.send_header("Content-type", "text/html")
         self.end_headers()
-        self.wfile.write(bytes("<html><head><title>OIDC Workflow Tool</title></head>", "utf-8"))
-        self.wfile.write(bytes("<body><p>The token has been received</p>", "utf-8"))
-        self.wfile.write(bytes("<p>You may now close this window.</p>", "utf-8"))
-        self.wfile.write(bytes("</body></html>", "utf-8"))
+        html_content = """<!DOCTYPE html>
+<html lang="en">
+<head>
+    <meta charset="UTF-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
+    <title>Authentication Successful - Gatehouse</title>
+    <!-- Best-effort CSS load from primary site -->
+    <link rel="stylesheet" href="{SIGN_URL}/static/css/main.css">
+    <style>
+        * {{
+            margin: 0;
+            padding: 0;
+            box-sizing: border-box;
+        }}
+        body {{
+            font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
+            background-color: #f0f4f8;
+            min-height: 100vh;
+            display: flex;
+            align-items: center;
+            justify-content: center;
+        }}
+        .card {{
+            background: white;
+            border-radius: 12px;
+            box-shadow: 0 4px 20px rgba(0, 0, 0, 0.08);
+            padding: 48px 40px;
+            text-align: center;
+            max-width: 400px;
+            width: 90%;
+        }}
+        .checkmark {{
+            width: 64px;
+            height: 64px;
+            background: #10b981;
+            border-radius: 50%;
+            display: flex;
+            align-items: center;
+            justify-content: center;
+            margin: 0 auto 24px;
+        }}
+        .checkmark svg {{
+            width: 32px;
+            height: 32px;
+            stroke: white;
+            stroke-width: 3;
+            fill: none;
+        }}
+        h1 {{
+            color: #1f2937;
+            font-size: 24px;
+            font-weight: 600;
+            margin-bottom: 12px;
+        }}
+        p {{
+            color: #6b7280;
+            font-size: 16px;
+            line-height: 1.5;
+        }}
+        .fallback {{
+            margin-top: 24px;
+            padding-top: 24px;
+            border-top: 1px solid #e5e7eb;
+            color: #9ca3af;
+            font-size: 14px;
+        }}
+    </style>
+</head>
+<body>
+    <div class="card">
+        <div class="checkmark">
+            <svg viewBox="0 0 24 24">
+                <polyline points="20 6 9 17 4 12"></polyline>
+            </svg>
+        </div>
+        <h1>Authentication Complete</h1>
+        <p>You can now return to the terminal.</p>
+        <p class="fallback">If this window doesn't close automatically, you can close it manually.</p>
+    </div>
+    <script>
+        setTimeout(function() {{
+            window.close();
+            if (window.innerHeight > 0) {{
+                document.querySelector('.fallback').textContent = 'Window refused to close. You may close this tab manually.';
+            }}
+        }}, 2000);
+    </script>
+</body>
+</html>""".format(SIGN_URL=SIGN_URL)
+        self.wfile.write(bytes(html_content, "utf-8"))
 
         parsed_url = urlparse(self.path)
         query_data = dict(parse_qsl(parsed_url.query))
