class: "custom"はZurb Foundation用。
select_form.html.erb
<%= form_for(@list, url: {action: :action_name},class: "custom") do |f| %>
<label>ラベル</label>
<%= f.select(:mode,
[["名前1","name1"],
["名前2","name2"],
class: "medium",
selected: "name2",
) %>
<% end %>