Можно в style.css
@import url("style.css");
или в function.php
function load_style_script(){
wp_enqueue_style('b_bootstrap', get_template_directory_uri() . '/css/bootstrap.min.css');
wp_enqueue_style('b_font-awesome', 'https://maxcdn.bootstrapcdn.com/font-awesome/4.4.0/css/font-awesome.min.css');
wp_enqueue_style('b_roboto-slab', 'https://fonts.googleapis.com/css?family=Roboto+Slab:400,300,700&subset=latin,cyrillic');
wp_enqueue_style('b_open-sans', 'https://fonts.googleapis.com/css?family=Open+Sans:400,300,600,700&subset=latin,cyrillic');
wp_enqueue_style('b_main', get_template_directory_uri() . '/css/main.css');
wp_enqueue_script('b_jquery', get_template_directory_uri() . '/js/jquery.min.js', array(), '',true);
wp_enqueue_script('b_bootstrap', get_template_directory_uri() . '/js/bootstrap.min.js', array(), '',true);
wp_enqueue_script('b_common', get_template_directory_uri() . '/js/common.js', array(), '',true);
}
add_action('wp_enqueue_scripts','load_style_script');