You can customize the “Popup” block inner using the Combo Blocks plugin. This article will help you to customize the Popup wrapper.
To begin, insert a “Popup” block.
Click on the “Inner” tab.
Here you can add some style to the Popup inner section. Click on the Styles Tab.
Now Click on Add Style button and add some styles. If you don’t know how to style an element, then read this document.