app/views/shared/_flash_messages.html.erb
<% flash.each do |type, message| %>
<div class="flash-application container-lg">
<%= content_tag :div, class: "flash #{primer_class_for(type)}" do %>
<span class="flash-close js-flash-close">
<%= octicon('x') %>
</span>
<%= sanitize(message) %>
<% end %>
</div>
<% end %>