- Requiring apps not to use custom colors alone won’t fix it. Any bespoke UI element an app includes is at risk of breakage when a custom theme redefines the entire world. https://blogs.gnome.org/tbernard/2018/10/15/restyling-apps-a...
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.
When Apple makes a future OS versions they are expected to avoid breaking apps.
The comparison here would be not to future iOS versions, but to arbitrary jailbreak tweaks that overhaul the look system-wide. You're not ensuring that will look good with your app ;)