Cole o código abaixo antes de ]]></b:skin>

#comments{overflow:hidden} 
#comments h4{display:inline;padding:10px;line-height:40px} 
#comments h4,.comments .comment-header,.comments .comment-thread.inline-thread .comment{position:relative}
#comments h4,.comments .continue a{background:#1e05d8} 
#comments h4,.comments .user a,.comments .continue a{font-size:16px} 
#comments h4,.comments .continue a{font-weight:normal;color:#fff} 
#comments h4:after{content:"";position:absolute;bottom:-10px;left:10px;border-top:10px solid #1e05d8;border-right:20px solid transparent;width:0;height:0;line-height:0}
#comments .avatar-image-container img{border:0} 
.comment-thread{color:#111} 
.comment-thread a{color:#777} 
.comment-thread ol{margin:0 0 20px} 
.comment-thread .comment-content a,.comments .user a,.comments .comment-thread.inline-thread .user a{color:#1e05d8}
.comments .avatar-image-container,.comments .avatar-image-container img{width:48px;max-width:48px;height:48px;max-height:48px}
.comments .comment-block,.comments .comments-content .comment-replies,.comments .comment-replybox-single{margin-left:60px}
.comments .comment-block,.comments .comment-thread.inline-thread .comment{border:1px solid #ddd;background:#f9f9f9;padding:10px}
.comments .comments-content .comment{margin:15px 0 0;padding:0;width:100%;line-height:1em}
.comments .comments-content .icon.blog-author{position:absolute;top:-12px;right:-12px;margin:0;background-image: url(http://4.bp.blogspot.com/-awl7HMjMgh4/T89YtINejTI/AAAAAAAAGmQ/XxcTVLKUAtI/s320/picresized_1338990142_comment-author-blue.gif);width:36px;height:36px}
.comments .comments-content .inline-thread{padding:0 0 0 20px} 
.comments .comments-content .comment-replies{margin-top:0} 
.comments .comment-content{padding:5px 0;line-height:1.4em} 
.comments .comment-thread.inline-thread{border-left:1px solid #ddd;background:transparent}
.comments .comment-thread.inline-thread .comment{width:auto} 
.comments .comment-thread.inline-thread .comment:after{content:"";position:absolute;top:10px;left:-20px;border-top:1px solid #ddd;width:10px;height:0px}
.comments .comment-thread.inline-thread .comment .comment-block{border:0;background:transparent;padding:0}
.comments .comment-thread.inline-thread .comment-block{margin-left:48px} 
.comments .comment-thread.inline-thread .user a{font-size:13px} 
.comments .comment-thread.inline-thread .avatar-image-container,.comments .comment-thread.inline-thread .avatar-image-container img{width:36px;max-width:36px;height:36px;max-height:36px}
.comments .continue{border-top:0;width:100%} 
.comments .continue a{padding:10px 0;text-align:center} 
.comment .continue{display:none} 
#comment-editor{width:103%!important} 
.comment-form{width:100%;max-width:100%}

E vá fazendo as modificações que achar necessárias, como o fundo do comentário, o formato do avatar, etc.

Dúvidas? Ask-me!

Obrigada ao Personalizar o Blogger pelo código.