functions.php

<?php

function custom_discourse_comment( $input ) {
    ob_start();
    ?>
	
	<div class="media">
		<div class="media-left">
			<a href="{comment_url}">
				<img class="media-object" src="{avatar_url}" alt="{username}" style="width:64px; height:64px;">
			</a>
		</div>
		<div class="media-body">
			<h4 class="media-heading">{username} <small><a href="{comment_url}">{comment_created_at}</a></small></h4>
			
			{comment_body}
						
		</div>
	</div>
	
	<?php
$output = ob_get_clean();
return $output;
}
add_filter( 'discourse_comment_html', 'custom_discourse_comment' );