This is a complete tutorial to teach you how to create a brand new login page without any plugin. This example show you about modifying the functions.php file and add new css selectors.

Resource Site:

http://www.1stwebdesigner.com/wordpress/customize-your-wordpress-backend-for-personal-branding/

Comment:
In this tutorial there’s a thing doesn’t work, you have to change to first function that is

<?php function custom_login() {
      echo 'wplogin/wplogin.css" ?>';
}
add_action('login_head', 'custom_login');

to be this…

<?php
function custom_login() {
     echo ' <link rel="stylesheet" id="wp-css"  href="'.get_bloginfo('template_directory').'/wplogin.css" type="text/css" media="all" />';
}
add_action('login_head', 'custom_login');
?>