undefined error in less file put under assets - ruby-on-rails

Question

I bought a theme from here, after I integrated it to my project.
Server raised the error
WARN: tilt autoloading 'less' in a non thread-safe way; explicit require 'less' suggested.
Completed 500 Internal Server Error in 262ms
ActionView::Template::Error (.box-shadow is undefined
(in /Users/hsu-wei-cheng/pixnet_hackathon/website/goood_theme/app/assets/ace-admin-theme/css/less/ace-nav.less)):
2: %html
3: %head
4: %title GooodTheme
5: = stylesheet_link_tag "application", media: "all", "data-turbolinks-track" => true
6: = javascript_include_tag "application", "data-turbolinks-track" => true
7: / Description, Keywords and Author
8: %meta{content: "Your description", name: "description"}/
app/views/layouts/application.html.haml:5:in `_app_views_layouts_application_html_haml___3827456286267197904_2191503380'
The contents in app/assets/ace-admin-theme/css link
I add two files for including all files under their folder
+++ b/app/assets/ace-admin-theme/css/application.css
## -0,0 +1,3 ##
+/*
+*= require_tree .
+*/
+++ b/app/assets/ace-admin-theme/js/application.js
## -0,0 +1 ##
+//= require_tree .
Edited two files
+++ b/app/assets/javascripts/application.js
## -13,5 +13,6 ##
+//= require_tree ../ace-admin-theme/js
+++ b/app/assets/stylesheets/application.css
+ *= require_tree ../ace-admin-theme/css
config/application.rb
config.assets.paths << Rails.root.join("app", "assets", "lenord-single-page-theme")
config.assets.paths << Rails.root.join("app", "assets", "ace-admin-theme")
The folder structure
.
├── avatars
├── css
│   ├── images
│   │   └── ie6
│   ├── img
│   ├── less
│   │   ├── bootstrap
│   │   └── ext
│   └── uncompressed
├── font
├── images
│   └── gallery
├── img
└── js
├── date-time
│   └── locales
├── flot
├── fuelux
│   └── data
├── jqGrid
│   └── i18n
├── markdown
├── uncompressed
│   ├── date-time
│   │   └── locales
│   ├── flot
│   ├── fuelux
│   │   └── data
│   ├── markdown
│   └── x-editable
└── x-editable
Gemfile
source 'https://rubygems.org'
# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '4.0.0'
# Use postgresql as the database for Active Record
gem 'pg'
# Use SCSS for stylesheets
gem 'sass-rails', '~> 4.0.0'
# Use Uglifier as compressor for JavaScript assets
gem 'uglifier', '>= 1.3.0'
# Use CoffeeScript for .js.coffee assets and views
gem 'coffee-rails', '~> 4.0.0'
# See https://github.com/sstephenson/execjs#readme for more supported runtimes
# gem 'therubyracer', platforms: :ruby
# Use jquery as the JavaScript library
gem 'jquery-rails'
# Turbolinks makes following links in your web application faster. Read more: https://github.com/rails/turbolinks
gem 'turbolinks'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '~> 1.2'
group :doc do
# bundle exec rake doc:rails generates the API under doc/api.
gem 'sdoc', require: false
end
# Use ActiveModel has_secure_password
# gem 'bcrypt-ruby', '~> 3.0.0'
# Use unicorn as the app server
# gem 'unicorn'
# Use Capistrano for deployment
# gem 'capistrano', group: :development
# Use debugger
# gem 'debugger', group: [:development, :test]
gem "bootstrap-sass", "~> 3.1.1"
gem "therubyracer"
gem "hirb-unicode"
gem "devise"
gem "will_paginate", "~> 3.0"
gem "rspec"
gem "awesome_print", require: "ap"
gem "tinymce-rails"
gem "simple_form"
gem "haml-rails"
gem "pg"
gem "taps"
gem "pry"
gem "pry-remote"
gem "pry-nav"
gem "analytics-ruby"
gem "bcrypt-ruby"
gem "bourbon"
gem "cancan"
group :development do
gem "rspec-rails"
gem "guard-rspec"
end
gem "sunspot_rails"
gem "sunspot_solr"
group :assets do
end
gem "simple-navigation"
gem "simple-navigation-bootstrap"
group :assets do
gem 'less'
end

Answer 1

You might find it easier to add the ace-admin-theme folder to your assets path.
# config/application.rb
config.assets.paths << Rails.root.join("app", "assets", "ace-admin-theme")