Trying to identify UI element that is not themed

[crawfordlong]

I have successfully edited the gtk.css files to add sidebar support that gradience does not currently provide, and it's working fine. I have one app, Newsflash (RSS reader), that isn't fully displaying the theme:

(This is one app displaying 3 columns: a sidebar, an article list, and a content panel.)

The Newsflash developer is pretty adamant about implementing libadwaita (and opposing theming), so I'm starting from an assumption that it doesn't have any weird UI elements. I guess my question is whether there is a way to determine the selectors that I need to add to the css file, whether the "list view" is a well-known UI control that gradience does not currently implement, etc.

Environment:

NixOS / ARM
Gradience installed via flatpak
Example app installed from nixpkgs (not flatpak)