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

app/lib/user_settings_decorator.rb

@@ -27,7 +27,6 @@ class UserSettingsDecorator
     user.settings['display_media']       = display_media_preference if change?('setting_display_media')
     user.settings['expand_spoilers']     = expand_spoilers_preference if change?('setting_expand_spoilers')
     user.settings['noindex']             = noindex_preference if change?('setting_noindex')
-    user.settings['theme']               = theme_preference if change?('setting_theme')
     user.settings['hide_network']        = hide_network_preference if change?('setting_hide_network')
     user.settings['aggregate_reblogs']   = aggregate_reblogs_preference if change?('setting_aggregate_reblogs')
     user.settings['group_in_home_feed']  = group_in_home_feed_preference if change?('setting_group_in_home_feed')
@@ -81,10 +80,6 @@ class UserSettingsDecorator
     boolean_cast_setting 'setting_hide_network'
   end
 
-  def theme_preference
-    settings['setting_theme']
-  end
-
   def default_language_preference
     settings['setting_default_language']
   end