added things for 'bootstrap' in Gemfile. do $ bundle install.

Your bundle is complete!
Use `bundle show [gemname]` to see where a bundled gem is installed.
Post-install message from twitter-bootstrap-rails:
Important: You may need to add a javascript runtime to your Gemfile in order for bootstrap's LESS files to compile to CSS.

**********************************************

ExecJS supports these runtimes:

therubyracer - Google V8 embedded within Ruby

therubyrhino - Mozilla Rhino embedded within JRuby

Node.js

Apple JavaScriptCore - Included with Mac OS X

Microsoft Windows Script Host (JScript)

**********************************************
result of "$ rails g bootstrap:install less".
result of "$ rails g bootstrap:layout application fluid".
result of "$ rails g simple_form:install --bootstrap".

===============================================================================

  Be sure to have a copy of the Bootstrap stylesheet available on your
  application, you can get it on http://twitter.github.com/bootstrap.

  Inside your views, use the 'simple_form_for' with one of the Bootstrap form
  classes, '.form-horizontal', '.form-inline', '.form-search' or
  '.form-vertical', as the following:

    = simple_form_for(@user, html: {class: 'form-horizontal' }) do |form|

===============================================================================
result of "$ rails g bootstrap:themed Products".