Users who have paid for a Theme, can use the full Tweak service to modify any of your saved themes:
Use the "Manager" page and click on the "Tweak" link for the saved Theme you want to edit.
This takes you in to the Tweak page where you can apply "tweaks" to your theme.
Users who have not paid for a Theme may not have access to the full Tweak service, but you can still apply tweaks to any Ecwid CSS theme loaded in your Ecwid Control Panel. Make sure you add the CSS generated by Tweak to the end of the existing CSS in your Ecwid CSS theme.