login-logo-custom-css.php

<?php
/*
Plugin Name: Login Logo Custom CSS
Description: Custom CSS for the login screen
Version: 0.1
License: GPL version 2 or any later version
Author: Mark Jaquith
Author URI: http://coveredwebservices.com/
*/

class CWS_Login_Logo_Custom_CSS_Plugin {
	static $instance;
	const HANDLE = 'cws-login-logo-custom-css';
	const VERSION = 1; // Increment this when you update the CSS

	public function __construct() {
		self::$instance = $this;
		add_action( 'login_head', array( $this, 'login_head' ), 1 );
	}

	public function login_head() {
		wp_register_style( self::HANDLE, plugin_dir_url( __FILE__ ) . 'login.css', array(), self::VERSION );
		wp_print_styles( self::HANDLE );
	}
}

// Bootstrap
new CWS_Login_Logo_Custom_CSS_Plugin;

login.css

/* CUSTOM CSS HERE */