Allow importing Discourse styles in custom stylesheets to modify vars.

app/views/common/_discourse_stylesheet.html.erb

@@ -6,9 +6,11 @@
   <% end %>
 <%- end %>
 
+<%= stylesheet_link_tag "plugins" %>
+
 <%- if staff? %>
   <%= stylesheet_link_tag "admin"%>
-<%-end%>
+<%- end %>
 
 <%- unless customization_disabled? %>
   <%= SiteCustomization.custom_stylesheet(session[:preview_style], mobile_view? ? :mobile : :desktop) %>