app/views/application/_flash_messages.html.erb
<% unless flash.empty? %>
<div class="container-fluid">
<div class="container">
<div class="row">
<div class="col-md-12">
<% flash.each do |message_type, message| %>
<%= content_tag :div, message,
class: "alert alert--header alert-primary alert-#{message_type} text-center" %>
<% end %>
</div>
</div>
</div>
</div>
<% end %>