js和css实现登录弹出并居中方法

<li id="create"><a href="#form" rel="external nofollow" ><span>创建相册</span></a></li>

<div class="background"></div>
  
<div id="form">
  <div class="fh">
    <h1>创建相册</h1>
    <a class="close">@@##@@</a>
  </div>
  ...
</div>
css代码
.background {
  display: none;
  position:fixed;
  top:0px;
  left:0px;
  width:100%;
  height:100%;
  background-color:#fff;
  background:-moz-radial-gradient(50% 50%, #fff, #000);/*gradient for firefox*/
  background:-webkit-radial-gradient(50% 50%, #fff, #000);/*new gradient for webkit */
  background:-webkit-gradient(radial, 0 50%, 0 50%, from(#fff), to(#000));/*the old grandient for webkit*/
  opacity:0.5;
  filter:Alpha(opacity=50);
}
#form {
  display: none;
  position:fixed;
  border: 1px solid #ccc;   
  background-color:white;
  top:30%;
  left:30%;
  width: auto;
  border-radius:15px;
  -moz-border-radius:15px;
  box-shadow:0 5px 27px rgba(0,0,0,0.3);
  -webkit-box-shadow:0 5px 27px rgba(0,0,0,0.3);
  -moz-box-shadow:0 5px 27px rgba(0,0,0,0.3);
}

function showForm() {
  var create = document.getElementById("create");
  var bg = document.getElementsByClassName("background")[0];
  var form = document.getElementById("form");
  var links = document.getElementsByClassName("close");
  for(var i=0;i<links.length;i++) {
    links[i].onclick = function() {
    form.style.display = "none";
    bg.style.display = "none";
    }
  }
  create.onclick = function() {
    form.style.display = "block";
    bg.style.display = "block";
  }