What I finally realized many months later is that it does work... it just doesn't change the default background. So, if you leave your main view's background on Default (transparent) and your if your Apple TV is set to light mode and you override the interface to use dark mode, everything will display in dark mode, except whatever the Apple TV's current background is will appear behind it.

Apple's documentation actually provides a Swift example of building a .apng that is quite old but still works just fine with a few modifications (I just tested it): https://developer.apple.com/library/archive/documentation/GraphicsImaging/Conceptual/ImageIOGuide/ikpg_dest/ikpg_dest.html the second example (listing 3-3) is what you're interested in. The reference to kUTTypePNG, which is deprecated, can be updated to UTType.png.identifier as CFString. Otherwise, you'll need to actually write the code to provide the UIImages. There's also this mysterious line in their example let radians = M_PI * 2.0 * Double(i) / Double(frameCount) which doesn't do anything and can be removed. My guess is the author was going to use it to render a rotating image but didn't bother to actually implement it. One other thing to note is that it seems like you may want to write the file with a ".png" extension even if it's an apng. I found instances where the ".apng" extension actually caused it to not work, but i'm not sure what the intended behavior is.

Having the same issue since updating to 15.3. I wonder if it's specific to cases (like mine) where the main app is just an empty container that holds the extension (you can't run it directly) Is there perhaps a way to configure the run scheme to build and deploy the main target app before running the messages extension? I've been trying but without success

Just tried 16 beta 2. It's broken in the same way as 15.3. This has made the workflow for developing message extensions almost impossible because i keep forgetting to do the extra of switching the scheme to run the base app, waiting for it to time out and then switching the scheme back to the message extension... and even worse it will happily execute the older build of the extension, so it's not always obvious that it didn't update.