iOS 18's icon theming looks rough. I don't give Google many props but their themed icons look fine, even if making good Android themed icons is quite tricky.
In Android, the backing colour is also changeable, so even a near-gray or a black can have a complimentary hue tint (such as in the above picture, which is not a pure black but a very dark reddish brown). Having non-pure darks makes things pop, and makes them feel immersive and really helps set a mood.
Also in Android, themed icons have to be distinct graphics that are made by developers for their apps, this enables them to make graphics that actually function in a monochromatic way.
What makes Apple's dark themed app icons truly bad is the absence of all of these kinds of design considerations.
- No colour sophistication at all. It's just pure grayscale gradient with a single bright colour sitting on top.
- The colours they showed in demos were often very basic primaries and had bad contrast.
- Some app icons seem adapted, but many (such as Find My) really don't seem to be. It seems that it just puts a colour filter over other ones.
I can't help but feel that this was rushed out the door. It takes me back to the app icon designs of the early 00's - early 10's, when people were still working out colour theory in app icons.