functions.php

<?php 
//If you're using this in your functions.php file, remove the opening <?php

//Replace style-login.css with the name of your custom CSS file
function my_custom_login_stylesheet() {
    wp_enqueue_style( 'custom-login', get_stylesheet_directory_uri() . '/style-login.css' );
}

//This loads the function above on the login page
add_action( 'login_enqueue_scripts', 'my_custom_login_stylesheet' );