Add ability to change an instance default theme from the administration panel (#7092) (#8381)

* Add default_settings class method to ScopedSettings

ScopedSettings was extended to use value of unscoped setting instead of
only using defaults set in config/settings.yml for selected settings.
This adds possibility for admins to set default values of users' settings,
for example default theme (as requested in #7092).

* Add ability to change an instance default theme

Closes #7092

spec/controllers/application_controller_spec.rb

@@ -92,6 +92,39 @@ describe ApplicationController, type: :controller do
     end
   end
 
+  describe 'helper_method :current_theme' do
+    it 'returns "default" when theme wasn\'t changed in admin settings' do
+      allow(Setting).to receive(:default_settings).and_return({'theme' => 'default'})
+
+      expect(controller.view_context.current_theme).to eq 'default'
+    end
+
+    it 'returns instances\'s theme when user is not signed in' do
+      allow(Setting).to receive(:[]).with('theme').and_return 'contrast'
+
+      expect(controller.view_context.current_theme).to eq 'contrast'
+    end
+
+    it 'returns instances\'s default theme when user didn\'t set theme' do
+      current_user = Fabricate(:user)
+      sign_in current_user
+
+      allow(Setting).to receive(:[]).with('theme').and_return 'contrast'
+
+      expect(controller.view_context.current_theme).to eq 'contrast'
+    end
+
+    it 'returns user\'s theme when it is set' do
+      current_user = Fabricate(:user)
+      current_user.settings['theme'] = 'mastodon-light'
+      sign_in current_user
+
+      allow(Setting).to receive(:[]).with('theme').and_return 'contrast'
+
+      expect(controller.view_context.current_theme).to eq 'mastodon-light'
+    end
+  end
+
   context 'ActionController::RoutingError' do
     subject do
       routes.draw { get 'routing_error' => 'anonymous#routing_error' }