FIX: Do not include theme variables in plugin SCSS, and fix register_css

lib/plugin/instance.rb

@@ -266,7 +266,7 @@ class Plugin::Instance
     automatic_assets.each do |path, contents|
       write_asset(path, contents)
       paths << path
-      assets << [path]
+      assets << [path, nil, directory_name]
     end
 
     delete_extra_automatic_assets(paths)