portal-theme

Follow the Linux desktop's light and dark setting in an egui window, through the XDG desktop portal.

winit reports the system theme on Windows and macOS and reports nothing on Linux, so an egui window there falls back to dark whatever the desktop is set to, and the window's own titlebar can draw light in the same frame. This crate asks the portal's org.freedesktop.appearance color-scheme once and then follows it for as long as the window is open. On the other two platforms it does nothing, since the toolkit is already answering and pinning a preference would replace a live answer with a stale one.

Using it

eframe::run_native("An application", options, Box::new(|cc| {
    portal_theme::follow(&cc.egui_ctx);
    Ok(Box::new(App::new()))
}))

One call where the context exists. Nothing is returned, nothing is kept by the caller, and the thread that watches for changes ends with the process.

Two rules

No preference is read as light. The portal defines 0 as no preference, 1 as prefer dark and 2 as prefer light. GNOME's Settings offers Light and Dark and spells them default and prefer-dark, so a person choosing Light produces 0 and never 2; prefer-light can only be set by hand through gsettings. Reading 0 as a reason to stay dark would give every GNOME user who chose Light a dark window, which is the defect this crate exists to prevent. Light is also what GTK does with its own default, so a window following this rule looks like its neighbours. A value the specification does not define is light for the same reason: it is not evidence that a person wants a dark window.

Nothing learned is not an answer. Where there is no session bus, no portal, or no appearance namespace, nothing is set and egui's own fallback stands. A machine without a portal behaves as if this crate were not there.

What it costs

Pure Rust, no build script, no C. The portal is reached over D-Bus through zbus, which an egui application on Linux already compiles for its screen reader support, and the crate's own source is forbid(unsafe_code). Off Linux it depends on egui alone and compiles to an empty function.

Licence

MIT.