function your_script_enqueue() {
wp_enqueue_script( 'bootstrap_js', 'https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/js/bootstrap.min.js', array('jquery'), NULL, true );
wp_enqueue_style( 'bootstrap_css', 'https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/css/bootstrap.min.css', false, NULL, 'all' );
wp_enqueue_style( 'custom_style', get_template_directory_uri() . '/css/your_style.css', array(), '1.0.0', 'all');
wp_enqueue_script( 'custom_js', get_template_directory_uri() . '/js/custom.js' );
}
add_action( 'wp_enqueue_scripts', 'your_script_enqueue' );