用jquery ui ,做登录框 ，但是却不会居中。请问该怎么设置 啊？
固定在了左上角。
代码如下：
用的是 jquery-ui-bootstrap
index.html
<script src="{{ static_url('js/jquery-1.8.3.min.js') }}" ></script>
<script src="{{ static_url('js/jquery-ui-1.8.16.custom.min.js') }}" ></script>
<script src="{{ static_url('js/global.js') }}" ></script>
<link href="{{ static_url('css/custom-theme/jquery-ui-1.8.16.custom.css') }}" rel="stylesheet" />
<link href="{{ static_url('bootstrap/bootstrap.css') }}" rel="stylesheet">
<a id="login-dialog" href="javascript:;">{{_('Login in ')}}</a>
<div id="auth-form" style="display:none;margin:auto;" title="用户登陆">
<p class="validateTips" style="height:20px">所有项为必填.</p>
<form action="">
<fieldset>
<br />
<label for="name">登陆名:</label>
<input type="text" name="name" id="name" class="text ui-widget-content ui-corner-all" /><br /><br />
<label for="password">密 码:</label>
<input type="password" name="password" id="password" class="text ui-widget-content ui-corner-all" /><br /><br />
</fieldset>
</form>
</div>
js:
var login_dialog = $(function() {
var email = $( "#email" ),
password = $( "#password" ),
allFields = $( [] ).add( email ).add( password );
$("#auth-form").dialog({
autoOpen: false,
height: 'auto',
width: 400,
modal: true,
stack:true,
title:"登录窗口",
buttons: {
"登陆": function() {
$.ajax({
url: "/account/login/",
type: "POST",
cache: false,