//居中方法1
position: absolute;
top: 0;
left: 0;
right: 0;
bottom: 0;
margin: auto;
346px;
height: 56px;
//居中方法2
position: absolute;
top: 50%;
left: 50%;
transform: translate(-50%, -50%);
//居中方法1
position: absolute;
top: 0;
left: 0;
right: 0;
bottom: 0;
margin: auto;
346px;
height: 56px;
//居中方法2
position: absolute;
top: 50%;
left: 50%;
transform: translate(-50%, -50%);