You can use this code:
add_action( 'admin_enqueue_scripts', 'load_admin_style' );
function load_admin_style() {
global $pagenow;
if ( 'post.php' === $pagenow && isset($_GET['post']) && 'YOURPOSTTYPE' === get_post_type( $_GET['post'] ) ) {
wp_enqueue_style( 'admin_css', get_template_directory_uri() . '/admin-style.css', false, '1.0.0' );
}
}