In Firebase Console, create a new A/B test with any name. In step two under
'Variants', click the 'Add Parameter' button and create a parameter named
color_scheme. Set its default value in the control group to
its value in 'Variant A' to
dark. In the third step, select any goal metric.
At this point, you can either run the sample and copy the printed Instance ID token from Xcode's console into the 'Test Device' section in Firebase Console, or simply run the experiment at a high percentage and reinstall the app until your app instance is in the A/B test.