The login body class filter allows body classes to be added to the login screen which is output via login_header().
The following example of this hook adds the .my-login-class body class to the login screen:
function login_classes( $classes ) {
$classes[] = 'my-login-class';
return $classes;
}
add_filter( 'login_body_class', 'login_classes' );
login_body_class is located in wp-login.php.
login_headerurl, login_headertitle, login_message