If a feature is in labs or developer settings, it's there because it's either an experimental feature that doesn't work consistently enough for Samsung to fully stand behind it as a feature, or it is a feature not recommended/not ready for general users. Several apps simply cannot be forced into dark mode, and Amazon is one of them.
I have that feature turned on. But I was under the impression that Samsung was going to make it permanent everywhere without doing anything. And apparently that's not the case. Is this coming out in one UI 8.5?
The feature attempts to apply dark mode to apps that don't support it, but cannot always do this successfully, which is why it is relegated to the aforementioned categories.
Nobody knows what the changelog for 8.5 will look like, as Samsung has not yet released info about that update. Rumors are rumors.
