CSS 3 Popup - absolutely positioned

HTML

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
    
    <head>
        <meta charset="utf-8">
        <title>iPlan Possessions - Login</title>
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <meta name="description" content="">
        <meta name="author" content="">
        <!-- Load JS here for greater good=============================-->
        <script type="text/javascript" src="<?php echo base_url(); ?>assets/js/admin/jquery-1.8.3.min.js"></script>
        <script type="text/javascript" src="<?php echo base_url(); ?>assets/js/admin/jquery-ui-1.10.3.custom.min.js"></script>
        <script type="text/javascript" src="<?php echo base_url(); ?>assets/js/admin/jquery.ui.touch-punch.min.js"></script>
        <script type="text/javascript" src="<?php echo base_url(); ?>assets/js/admin/bootstrap.min.js"></script>
        <script type="text/javascript" src="<?php echo base_url(); ?>assets/js/admin/bootstrap-select.js"></script>
        <script type="text/javascript" src="<?php echo base_url(); ?>assets/js/admin/bootstrap-switch.js"></script>
        <script type="text/javascript" src="<?php echo base_url(); ?>assets/js/html5shiv.js"></script>
        <script type="text/javascript" src="<?php echo base_url(); ?>assets/js/respond.min.js"></script>
        <!-- Load CSS here for greater good=============================-->
        <link href="<?php echo base_url(); ?>assets/css/admin/bootstrap.css" rel="stylesheet" type="text/css" />
        <link href="<?php echo base_url(); ?>assets/css/admin/flat-ui.css" rel="stylesheet" type="text/css" />
        <link href="<?php echo base_url(); ?>assets/css/admin/demo.css" rel="stylesheet" type="text/css" />
        
        <body>
            <div class="container">
                <div class="demo-headline"></div>
                <div class="login">
                    <div class="login-screen">
                        <div class="login-icon">
                           ...

CSS

/***********************
    Demo page styles
************************/
 body {
    width:100%;
    height:100%;
    background:url("../demo-images/bg.jpg") 0 0 repeat;
    margin:0;
    padding:0;
}
#page {
    margin:50px auto 0 auto;
    width: 960px;
    position: relative;
}
.demo {
    font-family:'Lobster', cursive;
    padding: 0 0 50px 0;
    text-align: center;
    color: #282828;
    text-shadow: 1px 1px #797979;
    font-size: 36px;
    width: auto;
    position: relative;
    margin: 0 auto;
}
.demo h1 {
    font-family: Arial, Helvetica, sans-serif;
    font-size: 11px;
    text-shadow: none;
    color: #a0a0a0;
    letter-spacing: 1px;
}