I need to auto fade the flash messages in ruby on rails.

My message code is:

```
<%= simple_form_for(@dashboard_user) do |f| %>
<% if @dashboard_user.errors.any? %>
<ul class="alert alert-danger">
    <% for message_error in @dashboard_user.errors.full_messages %>
    <li>
        <%= message_error %>
    </li>
    <% end %>
</ul>
<% end %>

```

How i auto fade these messages?