Removed all occurances of "theme" from backend, settings

• Removed:
- all occurances of "theme" from backend, settings

Theme is set in WebSettings through client. And set in <Display />.

app/lib/settings/scoped_settings.rb

@@ -2,9 +2,7 @@
 
 module Settings
   class ScopedSettings
-    DEFAULTING_TO_UNSCOPED = %w(
-      theme
-    ).freeze
+    DEFAULTING_TO_UNSCOPED = %w().freeze
 
     def initialize(object)
       @object = object