Styling comments_template() in index.php?

I’m running wordpress on a site for a friend and he wants comments to show up on his main blog page. I included the comments_template() below his posts on index.php but the CSS styling for comments.php seems to have disappeared. When I go into a post though (simple.php), the comments below retain their css styling. Is there a way I can make comments_template() on the main blog page retain their styling as well?

This is the code I used in the main blog page (index.php):

<?php
  $withcomments = "1";
  comments_template();
?>