How add backdrop-filter or any other css in Popup?

[zim]

How can I add backdrop-filter css in Popup, so that effect interacts with the page?
To create a glass effect.
I can add Custom CSS to Section class, but how to add Custom CSS to Template settings Popup?

[zim]

Was able to configure if in Template settings → Popup → CONTENT put Background color Transparency to 0 and setting up everything in Section

or
I find special popup class and add Custom CSS