<!DOCTYPE HTML>
<html lang="en-US">
<head>
<meta charset="UTF-8">
<title></title>
</head>
<body>
<style type="text/css">
button{
width:32px;
height:32px;
border-radius:20px;
outline:none;
color:green;
background:white;
border:solid 1px green;
cursor:pointer;
font-size:18px;
}
</style>
<button>+</button>
</body>
</html>
效果如图：
