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]

lib/guardian.rb

@@ -359,6 +359,8 @@ class Guardian
   end
 
   def allow_themes?(theme_ids)
+    return true if theme_ids.blank?
+
     if is_staff? && (theme_ids - Theme.theme_ids).blank?
       return true
     end

spec/services/user_updater_spec.rb

@@ -129,6 +129,29 @@ describe UserUpdater do
       expect(user.user_option.mailing_list_mode).to eq true
     end
 
+    it "filters theme_ids blank values before updating perferences" do
+      user = Fabricate(:user)
+      user.user_option.update!(theme_ids: [1])
+      updater = UserUpdater.new(acting_user, user)
+
+      updater.update(theme_ids: [""])
+      user.reload
+      expect(user.user_option.theme_ids).to eq([])
+
+      updater.update(theme_ids: [nil])
+      user.reload
+      expect(user.user_option.theme_ids).to eq([])
+
+      theme = Fabricate(:theme)
+      child = Fabricate(:theme)
+      theme.add_child_theme!(child)
+      theme.set_default!
+
+      updater.update(theme_ids: [theme.id.to_s, child.id.to_s, "", nil])
+      user.reload
+      expect(user.user_option.theme_ids).to eq([theme.id, child.id])
+    end
+
     context 'when sso overrides bio' do
       it 'does not change bio' do
         SiteSetting.sso_url = "https://www.example.com/sso"