You can edit any inactive theme.
Go to Layout > On the theme you want to edit, click on three dots and select the action you want to perform.
You can choose between:
- Change the name
- Customise the theme
- HTML editor
- Download theme
- Duplicate theme
- Change the layout
You can make the same changes on inactive themes as on the active ones.
This will create a new layout for your store without interrupting its operation.
During the initial setup, you can use the Cover theme and start building a mailing list before officially opening your store. How to add this layout you can read here.