I have checked that you are using the Astra theme. That’s not CSS but add some PHP code to change the text strings.
Add this code to the child-theme functions.php file to change the text strings.
add_filter('comment_form_defaults', 'wpastra_custom_comment_title', 20);
function wpastra_custom_comment_title( $defaults ){
$defaults['title_reply'] = __('Leave Your Thoughts Here...', 'astra');
return $defaults;
}
If you want to change the title tag, use this.
add_filter( 'comment_form_defaults', 'leave_a_comment_title_tag' );
function leave_a_comment_title_tag( $defaults ){
$defaults['title_reply_before'] = '<p id="reply-title" class="comment-reply-title">';
$defaults['title_reply_after'] = '</p>';
return $defaults;
}
Further any Astra related, support, I would highly suggest post it here https://www.ads-software.com/support/theme/astra/.