How do you add a class to a form_for helper in rails 3?

How do you add a class to a form_for helper in rails 3?

This is what i currently have in my form partial
<%= form_for @short, :html => {:class => "hello"} do |f| %>
and then in my css
#hello{
padding-top: 21px;
}
Does anyone know why this isnt working? I've seen this as an answer on
several questions on here.
Please let me know if i can include anything else that might lead to
figuring this out.
Css is included on the layout.html.erb page, I'm not really sure what else
I can rule out.
Thank you in advance