I see that I can define width and height for the popup in pixels. Is there a way to use other units, like % to make it for example 90% of the screen width? I assume I can use SCSS for it, but I want to apply it only to one popup, not all popups in the system, and I’m not sure how this css would work together with the popup width properties set in the settings.
Use the Properties section for the page. You can apply a class to the modal pop-up / pop-up page, which should allow you to modify the pop-up through SASS without affecting the rest of your application. Styles can also be directly added to your page.