Dans le fichier function.php du theme, ajouter:

via CDN bootstrap

 function theme_styles() {

 wp_enqueue_style( 'bootstrap_css', 'https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap-theme.min.css' );
 wp_enqueue_style( 'main_css', get_template_directory_uri() . '/style.css' );

 }

 add_action( 'wp_enqueue_scripts', 'theme_styles');

 function theme_js() {

 global $wp_scripts;

 wp_enqueue_script( 'bootstrap_js', 'https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js');
 wp_enqueue_script( 'my_custom_js', get_template_directory_uri() . '/js/scripts.js');

 }

 add_action( 'wp_enqueue_scripts', 'theme_js');

integre au theme

 function theme_styles() {

 wp_enqueue_style( 'bootstrap_css', get_template_directory_uri() . '/css/bootstrap-3.3.7.css' );
 wp_enqueue_style( 'main_css', get_template_directory_uri() . '/style.css' );

 }

 add_action( 'wp_enqueue_scripts', 'theme_styles');

 function theme_js() {

 global $wp_scripts;

 wp_enqueue_script( 'bootstrap_js', get_template_directory_uri() . '/js/bootstrap.min.js' );
 wp_enqueue_script( 'my_custom_js', get_template_directory_uri() . '/js/scripts.js');

 }

 add_action( 'wp_enqueue_scripts', 'theme_js');