I don't understand the question you are asking. Yes, if you configure GTK to load some CSS, it will do that. However that won't guarantee you see the correct styles, because CSS is technically not able to do that in all cases. If there are bugs in the the Pop_OS theme then it will continue to be broken, see the images at the bottom of the article...

Edit: I kind of alluded to this before but I want to make it clear, if you are trying to make it so the user can safely load themes without "special modifications to the app or compile process", then CSS is probably the wrong choice. It is not suited to do that, CSS is only safe to use when modifying the app itself, or if you are shipping a theme as a build-time dependency in the form of a library (i.e. libadwaita). So that's kind of why your question doesn't really follow to me.