I build bespoke UI elements for iOS apps all the time and building them in such a way that they look and function correctly not only across light/dark and accessibility modes but also future OS versions is not that difficult, and with how they’re built I don’t see them breaking even if iOS took on a radically different appearance. They might not match perfectly but usability will not be impacted.

If this is not reasonably possible with GTK, then it seems pretty clearly like a weakness of how GTK handles themes. Personally, I believe that CSS is ill-suited for the task and is responsible for many of the issues depicted in that blog post.