discourse/app/views/layouts/email_template.html.erb

<% if @disable_email_custom_styles %>
  <%= yield %>
  <% if defined?(html_body) %><%= html_body %><% end %>
<% else %>
  <%= email_html_template do %>
    <%= yield %>
    <% if defined?(html_body) %><%= html_body %><% end %>
  <% end %>
<% end %>