/**
 * Custom Login CSS
 *
 * @refer  https://millionclues.com/wordpress-tips/right-way-load-login-css-wordpress/
 */
function mc_load_custom_login_css() { 
  
  wp_enqueue_style( 'login-css', get_template_directory_uri() . '/assets/css/login-page.css' );
}
add_filter( 'login_enqueue_scripts' , 'mc_load_custom_login_css' );

Or you can use the Custom Login Admin Front-end CSS Plugin.