Force Browsers To Load Latest CSS And JS

functions.php

<?php
add_action( 'wp_enqueue_scripts', 'add_styles' );

function add_styles()
{
     $css_file = get_stylesheet_directory() . '/css/style.css';
     wp_enqueue_style( 'css-file', get_stylesheet_directory_uri().'/css/style.css', NULL, filemtime($css_file) );
}
?>