FIX: allow selecting site's default theme from preference

app/services/user_updater.rb

@@ -88,6 +88,7 @@ class UserUpdater
     # special handling for theme_id cause we need to bump a sequence number
     if attributes.key?(:theme_ids)
       user_guardian = Guardian.new(user)
+      attributes[:theme_ids].reject!(&:blank?)
       attributes[:theme_ids].map!(&:to_i)
       if user_guardian.allow_themes?(attributes[:theme_ids])
         user.user_option.theme_key_seq += 1 if user.user_option.theme_ids != attributes[:theme_ids]