A method for removing all default styles from WordPress login screen.

login-styles-removal.php

<?php
/**
 * Remove all WordPress-specific stylesheets from WordPress login screen
 */


/**
 * Force-remove login-related styles. Can't use the regular method of login_enqueue_scripts
 * due to wp_admin_css running before login_enqueue_scripts
 * 
 * @param WP_Styles $styles Current WP_Styles object containing dependencies
 * @return WP_Styles
 */
function ex_remove_login_styles(WP_Styles $styles)
{
    $styles->registered['login'] = array();
    return $styles;
}
add_action('wp_default_styles', 'ex_remove_login_styles');