structure.css -> default.css

Themes are optional, a theme overrides style in default.css

app/views/admin/designs/_form.html.erb

@@ -26,11 +26,11 @@
 <% end %> 
 </p>
 <p>
-<%= f.label "structure_css", t('admin.structure_css') %>
-<%  if @design.structure_css.blank? %>
-    <%= f.file_field :structure_css %>
+<%= f.label "default_css", t('admin.default_css') %>
+<%  if @design.default_css.blank? %>
+    <%= f.file_field :default_css %>
   <% else%>
-    <%= File.basename (@design.structure_css.url)  %>
+    <%= File.basename (@design.default_css.url)  %>
 <% end %> 
 </p>
 <p>