iced::system::theme_changes does not report system appearance changes on macOS 15.7

[mattly]

Is your issue REALLY a bug?

• My issue is indeed a bug!
• I am not crazy! I will not fill out this form just to ask a question or request a feature. Pinky promise.

Is there an existing issue for this?

• I have searched the existing issues.

Is this issue related to iced?

• My hardware is compatible and my graphics drivers are up-to-date.

What happened?

I am attempting to use iced::system::theme_changes to create a subscription to notify my app when the system switches between light and dark mode, so my app can follow. It is not working on macOS 15.7. I found a relevant discussion in the discord from last December, but could not find any resolution for that issue. I have a minimal reproduction working:

[package]
name = "iced-theme-changes"
version = "0.1.0"
edition = "2024"

[dependencies]
iced = "0.14.0"

use iced::{
    Element, Length, Subscription, Task, Theme, system,
    theme::Mode,
    widget::{container, text},
};

enum Message {
    ThemeModeChanged(Mode),
}

struct App {
    theme_light: Theme,
    theme_dark: Theme,
    mode: Mode,
}

impl App {
    fn new() -> Self {
        Self {
            theme_light: Theme::Light,
            theme_dark: Theme::Dark,
            mode: Mode::None,
        }
    }

    fn update(&mut self, msg: Message) -> Task<Message> {
        match msg {
            Message::ThemeModeChanged(mode) => self.mode = mode,
        };
        Task::none()
    }

    fn theme(&self) -> Theme {
        match self.mode {
            Mode::None => self.theme_light.clone(),
            Mode::Light => self.theme_light.clone(),
            Mode::Dark => self.theme_dark.clone(),
        }
    }

    fn subscription(&self) -> Subscription<Message> {
        system::theme_changes().map(|mode| {
            std::println!("theme changed! {mode:?}");
            Message::ThemeModeChanged(mode)
        })
    }

    fn view(&self) -> Element<'_, Message> {
        container(text!("Hello Theme Changes"))
            .center(Length::Fill)
            .into()
    }
}

fn main() -> Result<(), iced::Error> {
    iced::application(App::new, App::update, App::view)
        .subscription(App::subscription)
        .theme(App::theme)
        .run()
}

What is the expected behavior?

When I change from "Light" to "Dark" or vice-versa in the macOS Settings app's Appearance pane, it should print a message and change the appearance of the app.

Version

crates.io release

Operating System

macOS

Do you have any log output?