app/views/layouts/application.html.erb

<!doctype html>
<html lang="en">
  <head>
    <title><%= @cms_page.present? ? @cms_page.label : "Demo App" %></title>
    <!-- Required meta tags -->
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">

    <meta name="description" content="<%= cms_fragment_content("meta-description") %>">

    <!-- Bootstrap CSS -->
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/css/bootstrap.min.css" integrity="sha384-PsH8R72JQ3SOdhVi3uxftmaW6Vc51MKb0q5P2rRUpPvrszuE4W1povHYgTpBfshb" crossorigin="anonymous">

     <%= csrf_meta_tags %>
     <%= stylesheet_link_tag 'application', media: 'all', 'data-turbolinks-track': 'reload' %>
  </head>
  <body>

    <%= yield %>

    <!-- uncomment if you want to see what those variables contain
    <%= debug @cms_site %>
    <%= debug @cms_layout %>
    <%= debug @cms_page %>
    -->

    <!-- jQuery first, then Popper.js, then Bootstrap JS -->
    <script src="https://code.jquery.com/jquery-3.2.1.slim.min.js" integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.3/umd/popper.min.js" integrity="sha384-vFJXuSJphROIrBnz7yo7oB41mKfc8JzQZiCq4NCceLEaO4IHwicKwpJf9c9IpFgh" crossorigin="anonymous"></script>
    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/js/bootstrap.min.js" integrity="sha384-alpBpkh1PFOepccYVYDB4do5UnbKysX5WZXm3XxPqe5iKTfUKjNkCk9SaVuEZflJ" crossorigin="anonymous"></script>

    <%= javascript_include_tag 'application', 'data-turbolinks-track': 'reload' %>
  </body>
</html>