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 />.

config/settings.yml

@@ -20,7 +20,6 @@ defaults: &defaults
   preview_sensitive_media: false
   group_in_home_feed: true
   noindex: false
-  theme: 'default'
   aggregate_reblogs: true
   chat_messages:
     restrict_non_followers: true