简单的表单样式不起作用

我用启动了一个新的应用程序 simple_form 我的表格没有样式。

GemFile(GEM文件):

source 'https://rubygems.org'

# BACK
gem 'rails', '4.2.7.1'
gem 'pg', '~> 0.15'

# FRONT
gem 'coffee-rails', '~> 4.1.0'
gem 'jquery-rails'
gem 'jquery-ui-rails'
gem 'uglifier', '>= 1.3.0'
gem 'jbuilder', '~> 2.0'
gem 'simple_form'
gem 'slim'
gem 'bootstrap', '~> 4.1.1'
gem 'sprockets-rails', '>=2.3.2'

group :development, :test do
  # Call 'byebug' anywhere in the code to stop execution and get a debugger console
  gem 'byebug'
  gem 'rspec-rails', '~> 3.7'
end

group :development do
  # Access an IRB console on exception pages or by using <%= console %> in views
  gem 'web-console', '~> 2.0'
  gem 'rubocop'

  # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
  gem 'spring'
end

我的 app/assets/stylesheets/application.rb :

@import "bootstrap";

表单代码:

.row
  .offset-md-3.col-md-6
    = simple_form_for :user, url: sessions_path do |f|
      = f.text_field :username
      = f.password_field :password

      = f.submit 'Войти'

它会产生

<form novalidate="novalidate" class="simple_form user" action="/sessions" accept-charset="UTF-8" method="post">
  <input name="utf8" type="hidden" value="✓">
  <input type="hidden" name="authenticity_token" value="...">
  <input type="text" name="user[username]" id="user_username">
  <input type="password" name="user[password]" id="user_password">
  <input type="submit" name="commit" value="Войти">
</form>

看起来是这样的:

我能看到的是,它至少不会产生 <div class='form-group'> 每个输入的周围。 我已经跑了 rails generate simple_form:install --bootstrap . 并能提供所需的所有信息。

有什么意义?