Hi Julie -

You have a few options here. One is what you mentioned - you could add another site to your nation and build a theme out live there. You can also create a custom theme on your existing site but not switch to that site theme right away, so you can work with the theme files without any of the changes going live. To do this, when you click "create new custom theme" and you choose "clone existing theme" be sure to uncheck "switch to new theme immediately"

That will make it so you can work with those files and preview them while your live site still displays a different theme! When you're ready to switch to that theme, you would go to website - theme - switch to a custom theme and click on "switch to" next to the custom theme you've been working on. Let us know if you have any more questions on this at [email protected]!