login-customize.css

body.login{
  background-color: #fff;
  background-image: url(http://2013.tokyo.wordcamp.org/files/2013/06/bg_kabuki.png);
}
.login h1 a {
  background-image: url(http://2013.tokyo.wordcamp.org/files/2013/07/head_wapuu.png);
  background-size: 274px 300px;
}

.login h1 a {
background-size: 190px 181px;
background-position: top center;
background-repeat: no-repeat;
width: 190px;
height: 181px;
text-indent: -9999px;
outline: 0;
overflow: hidden;
padding-bottom: 15px;
display: block;
margin: 0 auto;
}

.login form {
margin-left: 8px;
padding: 26px 24px 46px;
font-weight: normal;
background: #fff;
border: none;
-webkit-box-shadow: none;
box-shadow: none;
}

#login {
padding: 20px 0;
}

.wp-core-ui .button-primary {
background-color: #f80016;
background-image: -webkit-gradient(linear,left top,left bottom,from(#f80016),to(#f80016));
background-image: -webkit-linear-gradient(top,#f80016,#f80016);
background-image: -moz-linear-gradient(top,#f80016,#f80016);
background-image: -ms-linear-gradient(top,#f80016,#f80016);
background-image: -o-linear-gradient(top,#f80016,#f80016);
background-image: linear-gradient(to bottom,#f80016,#f80016);
border-color: #f80016;
border-bottom-color: none;
-webkit-box-shadow: none;
box-shadow: none;
color: #fff;
text-decoration: none;
text-shadow: 0 1px 0 rgba(0,0,0,0.1);
}
.wp-core-ui .button-primary:hover {
background-color: #ff5742;
background-image: -webkit-gradient(linear,left top,left bottom,from(#ff5742),to(#ff5742));
background-image: -webkit-linear-gradient(top,#ff5742,#ff5742);
background-image: -moz-linear-gradient(top,#ff5742,#ff5742);
background-image: -ms-linear-gradient(top,#ff5742,#ff5742);
background-image: -o-linear-gradient(top,#ff5742,#ff5742);
background-image: linear-gradient(to bottom,#ff5742,#ff5742);
border-color: #ff5742;
border-bottom-color: none;
-webkit-box-shadow: inset 0 1px 0 rgba(120,200,230,0.5);
box-shadow: inset 0 1px 0 rgba(120,200,230,0.5);
color: #fff;
text-decoration: none;
text-shadow: 0 1px 0 rgba(0,0,0,0.1);
}

.login #nav a, .login #backtoblog a {
color: #fff!important;
}
.login #nav, .login #backtoblog {
text-shadow: #000 0 1px 0;
}

login-customize.php

<?php
/*
Plugin Name: Login Customize
*/
 
// ログイン画面でCSSを読み込み
function login_background_image() {
  echo '<link rel="stylesheet" type="text/css" href="' . plugins_url( 'login-customize.css' , __FILE__ ) .'" />';
} 
add_action('login_head', 'login_background_image');
 
// ログインフォームの下にコンテンツを追加
function add_footer_contents(){
  echo '';
}
add_action('login_footer', 'add_footer_contents');
 
// ロゴのリンク先を変更
function change_login_headerurl($login_header_url) {
  $login_header_url = 'http://2013.tokyo.wordcamp.org/';
  return $login_header_url;
}
add_filter('login_headerurl', 'change_login_headerurl');
 
// ロゴのtitle属性を変更
function change_login_headertitle($login_headertitle) {
  $login_headertitle = 'WordCamp Tokyo 2013 | 2013年9月14日 (土) 開催';
  return $login_headertitle;
}
add_filter('login_headertitle', 'change_login_headertitle');
 
// ログイン画面でjQueryを追加
function add_jquery_to_login(){
  wp_enqueue_script('jquery');
}
add_action('login_enqueue_scripts','add_jquery_to_login');
 
// ログインフォームをdivで囲む
function add_script_to_login(){
  echo '<script>jQuery("#loginform").wrap("<div class=\'hoge\'></div>");</script>';
}
add_action('login_footer','add_script_to_login');