Umbraco Engage takes care of applying the correct styles to the pages part of your A/B tests.
The A/B Test will automatically be rendered in your front end. If you have set up a Single page A/B test Umbraco Engage will automatically render the correct properties.
If you have set up one of the other tests and added some CSS and JavaScript this will automatically be added to your webpage.
You do not have to do anything to render the A/B test. If you want to run a more advanced A/B test that requires more coding, you can always do that.