While you’re setting up your store, you can make it invisible to your customers.
You have two possibilities:
Setting up a password
Go to Layout > Click on Edit at the top right side of the page
Once the store is password protected, it will look like this:
If you want, you can add a message to your customers, also on the edit section:
Activating the Cover theme
Cover (Launch) – this theme will let you create a mailing list before the official launch of your store – Check this theme out.
Install the Cover theme. Then go to Layout > and under Installed themes, you should see the theme you just installed. Click on the button next to Activate.
Read the article: How to install and activate a theme?
Meanwhile, you can work on your store’s layout on one of your inactive themes. Learn more about how to edit inactive themes.