Bootstrap: Import file not found or unreadable

I already found several topics about my problem, but I did not understand how this should be fixed.

I followed the http://ruby.railstutorial.org/chapters/filling-in-the-layout#fnref-5_5 training course and got stuck importing the boot file into app / assets / stylesheets / custom.css.scss.

This is my gemfile:

source 'https://rubygems.org'
gem 'bootstrap-sass', '2.0.4'
gem 'rails', '3.2.8'

group :development, :test do
gem 'sqlite3', '1.3.5'
gem 'rspec-rails', '2.11.0'
end

Gems used only for assets and not required
in production environments by default.
group :assets do
gem 'sass-rails', '3.2.5'
gem 'coffee-rails', '3.2.2'
gem 'uglifier', '1.2.3'
end

gem 'jquery-rails', '2.0.2'

group :test do
gem 'capybara', '1.1.2'
end

group :production do
gem 'pg', '0.12.2'
end

And what error do I get at localhost / sample_app / home

Sass::SyntaxError in Static_pages#home

Showing c:/Users/katja/Arbeit/rails_projects/sample_app/app/views/layouts/application.html.erb where line #5 raised:

File to import not found or unreadable: bootstrap.
Load path: Sass::Rails::Importer(c:/Users/katja/Arbeit/rails_projects/sample_app/app/assets/stylesheets/custom.css.scss)
(in c:/Users/katja/Arbeit/rails_projects/sample_app/app/assets/stylesheets/custom.css.scss)
Extracted source (around line #5):

2: 
3:

4: <%= full_title(yield(:title)) %>
5: <%= stylesheet_link_tag "application", media: "all" %>
6: <%= javascript_include_tag "application" %>
7: <%= csrf_meta_tags %>
8: <!--[if lt IE 9]>
Rails.root: c:/Users/katja/Arbeit/rails_projects/sample_app
Application Trace | Framework Trace | Full Trace
app/assets/stylesheets/custom.css.scss:1
app/views/layouts/application.html.erb:5:in `app_views_layouts_application_html_erb_16020705_20168520'

I am using Windows and GitBash. I already tried to remove tmp / cache, this did not help. Everything works fine, but when I add @import "bootstrap"; to cusom.css.scss, I get an error.