Custom HTML/CSS in pop-up plugin not working

hi there

i am trying to use pop-up plugin, and window with pop-up is showing on the website, but i can not add any custom html/css to it. I add css at the bottom in 'custom css' filed, btu none of changes are applied to pop-up.

As I want to replace the whole popup with my custom html/css - is there a way I can just replace the whole lot? Like not adding css, but just inserting my whole html with css into one field that could replace the whole look of your popup? How can i do this?

Screenshot attached. Thanks