Preview Theme ID No Longer Works?

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Tourist
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How do I display an alternate theme?   The preview_theme_id parameter no longer works.

It seems like if I click preview theme from the admin, a cookie or token is set because it becomes the default when I check the front end logged in.

Also, I notice that in customize theme homepage editor, the iframe Shopify uses has an empty preview_theme_id parameter.   If I open the iframe in a new tab, the theme switches to the default theme.  Exact same URL - different theme.

Not sure why Shopify is making the theme preview so cryptic...  why remove the preview_theme_id functionality?