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I don't like the appearance?

Please customize it by editing ajaxcomment/comment.css and ajaxcomment/comments.php

I use the default CSS, but it isn't the same as the demo site?

Maybe your template/style.css define CSS of the comment form, and they conflict with default one. Delete them.

I want the comments sorted by post date reversely

In ajaxcomment/comments.php, uncomment line // $comments = array_reverse($comments);