Custom css for popups

Hello,

First of all, thank you for your work on this plugin!
I would like to customize the design of the popups, such as add a border-radius.
I tried to add css properties to the style.css file of my child theme by targeting .et_pb_section.popup, but no change.
I then directly modified the css file of your plugin. Changes in existing properties were taken into consideration (the color of the background for exemple) but not those that were not originally present.

Could you give me a solution?

In advance, thank you!