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/controllers/settings/preferences_controller.rb

@@ -44,7 +44,6 @@ class Settings::PreferencesController < Settings::BaseController
       :setting_display_media,
       :setting_expand_spoilers,
       :setting_noindex,
-      :setting_theme,
       :setting_hide_network,
       :setting_aggregate_reblogs,
       :setting_group_in_home_feed,