总结：为所有被选中的 input 元素直接兄弟元素span设置背景色：
input:checked +span{ background-color: pink; }
小点：点击label就会触发input控件，所以可以把label设置溢出隐藏把input藏起来，内部再设置span标签绝对定位填充
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<style>
label{
position: relative;
display: inline-block;
width: 100px;
height: 100px;
border: 1px solid;
border-radius: 50% 50%;
overflow: hidden;
}
label>input{
position: absolute;
left: -50%;
top: -50%;
}
label>span{
display: block;
position: absolute;
top: 0;
right: 0;
left: 0;
bottom: 0;
}
input:checked +span{
background-color: pink;
}
</style>
</head>
<body>
<label><input type="radio" name="testInput"><span></span></label>
<label><input type="radio" name="testInput"><span></span></label>
<label><input type="radio" name="testInput"><span></span></label>
</body>
</html>
