functions.php

<?php 

function theme_styles()
{
    wp_enqueue_style( 'theme_css', get_template_directory_uri() . '/public/css/app.css' );
}
add_action( 'wp_enqueue_scripts', 'theme_styles' );

function theme_js()
{
    global $wp_scripts;

    wp_register_script( 'html5_shiv', 'https://oss.maxcdn.com/libs/html5shiv/3.7.0/html5shiv.js', '', '', false );
	wp_register_script( 'respond_js', 'https://oss.maxcdn.com/libs/respond.js/1.4.2/respond.min.js', '', '', false );
    $wp_scripts->add_data( 'html5_shiv', 'conditional', 'lt IE 9' );
	$wp_scripts->add_data( 'respond_js', 'conditional', 'lt IE 9' );

    wp_enqueue_script( 'theme_js', get_template_directory_uri() . '/public/js/app.js', array('jquery'), '', true );
}
add_action( 'wp_enqueue_scripts', 'theme_js' );