First access your main website by clicking on either Portfolio or Builder.
From there, hover over design and click on custom css/js.
Enter your custom code in the boxes provided. You may see code in this area already that is automatically added with some templates. You can edit the existing code or enter your own.
After your code is entered, click on save changes.
If you're using the Portfolio, make sure to go to publish and click on publish now when you're ready for your changes to be live.