在 functions.php 最后一个？>前加入以下代码：

//自动加勾选栏
function  add_checkbox (){
	echo '<input type="checkbox" name="comment_mail_notify" id="comment_mail_notify" value="comment_mail_notify" checked="checked" class="chk_1"/><label for="comment_mail_notify">有人回复时邮件通知我</label>';

或自行找到评论模板添加如下代码：

<input type="checkbox" name="comment_mail_notify" id="comment_mail_notify" value="comment_mail_notify" checked="checked" class="chk_1"/>
<label for="comment_mail_notify"></label>

以上代码根据自己的模板来修改，实际效果只是简单的勾选框，但博主喜欢花哨一点所以进行了美化：

css美化 带ON、OFF的效果：

/*把原来的复选框给隐藏*/
.chk_1 { 
    display: none; 
}
 
/*美化复选框其把握二种状态：勾选与未勾选二种的样式就行*/
.chk_1 + label {
	display: inline-block;
	position: relative;
	color: #aaa;
	margin-left: 20px;
	padding-left: 65px;
}
 
.chk_1:checked + label {
	color: #000;
}
 
.chk_1 + label::before {
	content: '';
	position: absolute;
	left: 5px;
	top: 0px;
	width: 18px;
	height: 18px;
	border-radius: 50%;
	background: #fff;
	z-index: 2;
	transition: all .3s;
}
 
.chk_1:checked + label::before {
	left: 37px;
}
 
.chk_1 + label::after {
	content: 'OFF';
	position: absolute;
	top: -3px;
	left: 0;
	width: 60px;
	height: 25px;
	border-radius: 25px;
	background: #ED6F6F;
	color: #fff;
	line-height: 25px;
	padding-left: 28px;
	font-size: 12px;
	transition: all .3s;
}
 
.chk_1:checked + label::after {
	content: 'ON';
	padding-left: 10px;
	background: #4FB845;
}

适用于本站设计风格的css样式如下：

/*邮件通知勾选框*/
/*把原来的复选框给隐藏*/
.chk_1 { 
    display: none; 
}
 
/*美化复选框其把握二种状态：勾选与未勾选二种的样式就行*/
.chk_1 + label {
	display: inline-block;
	position: relative;
	color: #aaa;
	margin-left: 20px;
	padding-left: 60px;
}
 
.chk_1:checked + label {
	color: #000;
}
 
.chk_1 + label::before {
	content: '';
	position: absolute;
	left: 5px;
	top: -12px;
	width: 18px;
	height: 18px;
	border-radius: 50%;
	background: #fff;
	z-index: 2;
	transition: all .3s;
}
 
.chk_1:checked + label::before {
	left: 37px;
}
 
.chk_1 + label::after {
	content: 'OFF';
	position: absolute;
	top: -18px;
	left: 0;
	width: 62px;
	height: 32px;
	border-radius: 25px;
	background: #ED6F6F;
	color: #fff;
	line-height: 25px;
	padding-left: 28px;
	font-size: 12px;
	transition: all .3s;
        padding-top: 5px;
        border-radius: 7px;
        line-height: 22px;
}
 
.chk_1:checked + label::after {
	content: 'ON';
        line-height: 22px;
        border-radius: 7px;
	padding-left: 10px;
        padding-top: 5px;
	background: #20abff;
}
/*鼠标手势*/
.chk_1 + label:hover{cursor:pointer}