theme_rewrites.php

<?php
function add_filters($tags, $function) {
  foreach($tags as $tag) {
    add_filter($tag, $function);
  }
}

function roots_rewrites() {
  /**
   * Define helper constants
   */
  $get_theme_name = explode('/themes/', get_template_directory());

  define('RELATIVE_PLUGIN_PATH',  str_replace(home_url() . '/', '', plugins_url()));
  define('RELATIVE_CONTENT_PATH', str_replace(home_url() . '/', '', content_url()));
  define('THEME_NAME',            next($get_theme_name));
  define('THEME_PATH',            RELATIVE_CONTENT_PATH . '/themes/' . THEME_NAME);

  /**
   * Rewrites do not happen for multisite installations or child themes
   *
   * Rewrite:
   *   /wp-content/themes/themename/assets/css/ to /assets/css/
   *   /wp-content/themes/themename/assets/js/  to /assets/js/
   *   /wp-content/themes/themename/assets/img/ to /assets/img/
   *   /wp-content/plugins/                     to /plugins/
   *
   * If you aren't using Apache, Nginx configuration settings can be found in the README
   */
  function roots_add_rewrites($content) {
    global $wp_rewrite;
    $roots_new_non_wp_rules = array(
      'assets/css/(.*)'          => THEME_PATH . '/assets/css/$1',
      'assets/js/(.*)'          => THEME_PATH . '/assets/js/$1',
      'assets/img/(.*)'          => THEME_PATH . '/assets/img/$1',
      'assets/fonts/(.*)'          => THEME_PATH . '/assets/fonts/$1',
      'plugins/(.*)'         => RELATIVE_PLUGIN_PATH . '/$1'
    );
    $wp_rewrite->non_wp_rules = array_merge($wp_rewrite->non_wp_rules, $roots_new_non_wp_rules);
    return $content;
  }

  function roots_clean_urls($content) {
    if (strpos($content, RELATIVE_PLUGIN_PATH) > 0) {
      return str_replace('/' . RELATIVE_PLUGIN_PATH,  '/plugins', $content);
    } else {
      return str_replace('/' . THEME_PATH, '', $content);
    }
  }

  if (!is_multisite() && !is_child_theme()) {
    add_action('generate_rewrite_rules', 'roots_add_rewrites');

    if (!is_admin()) {
      $tags = array(
        'plugins_url',
        'bloginfo',
        'stylesheet_directory_uri',
        'template_directory_uri',
        'script_loader_src',
        'style_loader_src'
      );

      add_filters($tags, 'roots_clean_urls');
    }
  }
}
add_action('after_setup_theme', 'roots_rewrites');