How can I preview the subscription widgets in a duplicate theme in order to test it before going live?

In your Seal Subscription app go to Settings > General settings > Advanced settings. Copy your store URL (or domain) and paste it in the text box Disable app for specific urls. Note that this will disable the app on all pages that contain the domain you added to this field, essentially disabling it in your whole store. Save settings. Go to Online store > at the theme you want to tests choose Actions > Preview. On the bottom of the page a pop-up will appear with a box Share preview, click on the box and copy and paste the URL into your browser. You are now previewing your shop and you can check and see how the Seal Subscription widgets look in your new theme.