How can I show the widgets only in my new unpublished theme?
To show the widgets only in a specific theme, you can turn on the option to show the widgets only if the HTML snippet is present in your theme. This can be turned on in Seal Subscriptions -> Settings -> General Settings -> Advanced (at the bottom of the page).
Then, just install the snippets in a new theme according to our instructions here: https://www.sealsubscriptions.com/tutorials/manual-installation