bootstrap-css

// Register Style
function custom_styles() {

	wp_register_style( 'bootstrap-css', 'https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css', false, '4.1.3' );
	wp_enqueue_style( 'bootstrap-css' );

}
add_action( 'wp_enqueue_scripts', 'custom_styles' );