Tammy,

I would recommend cloning the site and working on the theme on the cloned site. With this draft website, you'll want to update the site settings - specifically, you may want to change who can view the site by toggling 'who can view this site' under the Website tab > Site Settings.  You'll also want to uncheck the box for allowing search engines to discover the site also.  This is done under the Website tab > Site Settings > and then uncheck the box for 'Allow Search Engines'. Then you can create a custom theme and test out how the design will look before launching the new site.

If you plan significant content changes, once the second site is ready to launch, you can make that site the primary site for your nation by going to Settings > Basics, change Primary website from drop-down menu and then flip your custom domain to connect to the new site by navigating to Settings > Domains.

Alternatively, you can choose to work with an Architect to design the site. An architect can create a custom theme and share it with your nation.