We are looking for an efficient way to make changes on our theme without affecting the site in production.
Thanks for the question
Unfortunately, at the moment there is not a way to do that within a single eCom shop.
What can be done is to simply apply for a trial account, and use that as a testing grounds for your customization. This process is described in our Theme Developer's Guide, which I'd recommend as a resource when coding your theme.
Our eCom support team is able to install a customized theme from one shop to another, once you're ready to go live with your changes. When ready, simply open a support ticket with us!
Additionally, as the shop would be a stand-alone account, you could use an export from your live account to add products to your trial shop. See more on that here: Import tool overview