After using bulma for some time, I moved on to an even easier framework, called Simple.css. All you have to do is to include it:

<link rel="stylesheet" href="https://cdn.simplecss.org/simple.min.css">

A typical application.html.erb page would look like this:

<!DOCTYPE html>
<html>
  <head>
    <title><%= content_for(:title) || "Title App" %></title>

    <%= stylesheet_link_tag "application" %>
    <%= stylesheet_link_tag "https://cdn.simplecss.org/simple.min.css" %>

  </head>

  <body>
    <header>
      <nav>
        <ul>
          <li><%= link_to :home, root_path %>
        </ul>
      </nav>
    </header>
    <main>
      <%= yield %>
    </main>
    <footer>
      Copyright © Greinersoft
    </footer>
  </body>
</html>