My name is Jade, I'm with the Support Team at Shopify!
The Debut Theme does not have the pop-up feature as part of the theme. You can use a third party app such as Popup Upsell on Exit & Visit or Better Coupon Box - Free Email Popups to create popups on your website to promote sales and discount codes. Both apps have a free version and a paid version that allows you to integrate with MailChimp in order to add the feature of getting email sign ups. If you are more interested in getting email sign ups, you may prefer to use the app MailMunch - Email Popup.
You can also enable newsletter signup on your website. Login to your Shopify admin > click Online Store > Customize Theme > click Footer on the left side > check the box for Show newsletter signup and click Save.
Take Care ?