Hi there, what are you looking to put in the popup? Popups aren’t a theme-specific feature, so they are available in any theme with a widget area. The two I can think of are:
– MailChimp signup popup widget – https://en.support.wordpress.com/mailchimp/
– Cookie and privacy banner widget – https://en.support.wordpress.com/cookie-widget/ You may customize the content of this banner to say anything you’d like, so it’s pretty flexible.
Let me know if one of these will do the trick.