function custom_login() {
    echo '<link rel="stylesheet" type="text/css" href="' . get_bloginfo('template_directory') . '/custom-login.css" />';
}
add_action('login_head', 'custom_login');

html,body,#wpbody,.form-table .pre{background:url(http://s.014.cc/dot.gif) repeat;font-size:13px;font-family:Microsoft Jhenghei;}
a{text-decoration:none;}
h1 a{display:none;}
form{border:none;box-shadow:0 0 10px #c0c0c0;-moz-box-shadow:0 0 10px #c0c0c0;-webkit-box-shadow:0 0 10px #c0c0c0;}
#login{margin-top:150px;}
#nav{position:absolute;text-shadow:none;filter:alpha(opacity=0) ; -moz-opacity:0; opacity:0;}
#nav:hover{filter:alpha(opacity=100) ; -moz-opacity:1; opacity:1;}
.login #nav a{background-color:#EDEDED;color:#333 !important;padding: 3px 4px;}
.login #nav a:hover{color:#333 !important;}
input.button-primary, button.button-primary, a.button-primary,input.button-primary:hover, button.button-primary:hover, a.button-primary:hover{border:none;background:none;color:#333;font-weight:normal;background-color:#e1e1e1;text-shadow:none;font-size:12px;moz-border-radius:0;-webkit-border-radius:0;border-radius:0;-moz-box-shadow:none;-webkit-box-shadow:none;box-shadow:none;}
.login .message{background-color:#F0F0F0;border:0;box-shadow:0 0 10px #c0c0c0;-moz-box-shadow:0 0 10px #c0c0c0;-webkit-box-shadow:0 0 10px #c0c0c0;}

这个是rinkaiten.com博主的css，只是一张小图片，填充作为背景。

html,body,#wpbody,.form-table .pre{background:#F8F0E8;font-size:13px;font-family:微软雅黑,宋体;}
a{text-decoration:none;}
body.login,body{border:none;}
h1 a{display:none;}
#login{width:516px;position:relative;margin:50px auto;}
form{width:280px;height:166px;padding:181px 118px 102px;background:url('http://s.014.cc/wp-login.jpg') no-repeat;margin:0;border:none;-moz-border-radius:0;-webkit-border-radius:0;border-radius:0;-moz-box-shadow:none;-webkit-box-shadow:none;box-shadow:none;}
#user_pass,#user_login,#user_email{width:271px;height:30px;margin-bottom:10px;}
#nav{left:250px;margin:0;padding:0;position:absolute;bottom:105px;text-shadow:none;}
form .forgetmenot{position:absolute;bottom:105px;}
label,form .forgetmenot label{font-size:13px;color:#fff;}
.login #nav a{color:#fff !important;}
input.button-primary, button.button-primary, a.button-primary{background:none;border:none;text-shadow:none;margin:0;padding:0;font-size:13px;font-family:微软雅黑,宋体;}
.submit{position:absolute;bottom:105px;right:120px;}
input.button-primary:active, button.button-primary:active, a.button-primary:active {background:none;}

------------------------------------

另外在rinkaiten.com发现博客的背景是这样弄的

<style type="text/css" media="screen" >
body {background-image:url(http://s.014.cc/dot.gif);background-repeat:repeat;background-attachment:scroll;background-color:#FFFFFF;}
body,ul.top-navigation li.current_page_item a,#side h2, #side h3 {color:#000000;}
a, #logo .description {color:#000000;}
#logo h1 a {text-decoration:none;border-bottom:1px solid #000000;}
#side_base {background-color:#FFFFFF;border-left:1px solid #dbdbdb;}
#side h2, #side h3 {border-bottom:1px solid #dbdbdb;}
</style>

via:http://wange.im/wordpress-custom-login.html