You are able to create and manage your own popup design with HTML or CSS & JS editor. All you need to do is add a new creative, select one of the basic templates or custom, and customize it.
In case you wish to have a custom template, you need to choose the appropriate template for this purpose. All you need to do is add a new creative, select the custom template for desktop, and build your unique popup.
Please find the HTML guide here. It helps to build the campaign with HTML.
If you wish to create a new custom mobile template select the custom template for mobile: