fix: add missing theme-manager script to contributing page

Author: devanshi14malhotra

frontend/pages/contributing.html

@@ -176,6 +176,7 @@ <h2>Style Guide</h2>
     <button id="backToTop" aria-label="Back to top">↑</button>
   </div>
 
+  <script src="../js/theme-manager.js"></script>
   <script src="../js/contributing.js"></script>
 </body>
 </html>