Hide dark mode setting in OSS version

Summary: See https://github.com/facebook/flipper/issues/1755, we did hide the Sandy option in OSS, but not the dark mode option. I intend to enable & switch to Sandy in OSS after the holidays, to make it easier to deal with any possible fallout.

Reviewed By: nikoant

Differential Revision: D25460906

fbshipit-source-id: b80d751f2ff435775d08d17effb81e56db738d4d

desktop/app/src/chrome/SettingsSheet.tsx

@@ -29,6 +29,7 @@ import SandySettingsPanel from '../fb-stubs/SandySettingsPanel';
 import {reportUsage} from '../utils/metrics';
 import {Modal, message} from 'antd';
 import {Layout, withTrackingScope, _NuxManagerContext} from 'flipper-plugin';
+import GK from '../fb-stubs/GK';
 
 const Container = styled(FlexColumn)({
   padding: 20,
@@ -254,7 +255,7 @@ class SettingsSheet extends Component<Props, State> {
             });
           }}
         />
-        {!disableSandy && (
+        {GK.get('flipper_sandy') && !disableSandy && (
           <ToggledSection
             label="Enable dark theme (experimental)"
             toggled={darkMode}