input[type="checkbox"] {
appearance:none;
-webkit-appearance:none;
-moz-appearance:none;
-ms-appearance:none;
-o-appearance:none;
width:14px;
height:14px;
border:1px solid #999;
border-radius:2px;
line-height:14px;
text-align:center;
}

input[type="checkbox"]:checked {
background:#BF040E;
border:none;
}

input[type="checkbox"]:checked:after {
content:"\2713";
color:#FFF;
}

<div style="display:flex;align-items:center">
<input name="Checkbox" type="checkbox"> 
<label>&nbsp;自动登录</label>
</div>

效果如下：

注意：content属性值要用引号引起来。