fix(linux): follow the desktop portal color scheme

CHANGELOG.md

@@ -8,6 +8,10 @@ separately in [CHANGELOG-DEPS.md](CHANGELOG-DEPS.md).
 
 ## Unreleased
 
+- **System theme follows the Linux desktop.** Ferail reads GPUI’s portal-backed
+  appearance instead of requiring GNOME settings schemas, and updates open
+  windows when the desktop switches between light and dark.
+
 - **Going up to a parent folder selects the folder you came from and scrolls
   it into view, whichever way you go up.** Backspace and Back already did
   this; clicking a breadcrumb segment, a folder in the sidebar tree, or typing

crates/ferail-gpui/src/boot.rs

@@ -221,31 +221,53 @@ pub fn run_gui(args: screenshot::Args) {
         //   1. `--theme {light,dark}` CLI flag
         //   2. `FERAIL_THEME` env var (light / dark / system)
         //   3. Persisted `theme_pref` in app_state
-        //   4. macOS Appearance via `system_is_dark()`
+        //   4. System appearance
         let env_theme = std::env::var("FERAIL_THEME").ok().map(|s| s.to_lowercase());
         let persisted_theme = crate::app_state::load().theme_pref;
         let resolve_string = |s: &str| -> Option<gpui_component::ThemeMode> {
             match s {
                 "light" => Some(gpui_component::ThemeMode::Light),
                 "dark" => Some(gpui_component::ThemeMode::Dark),
-                "system" | "auto" => Some(if crate::platform_shell::system_is_dark() {
-                    gpui_component::ThemeMode::Dark
-                } else {
-                    gpui_component::ThemeMode::Light
-                }),
+                "system" | "auto" => Some(crate::settings::ThemePref::System.resolve(cx)),
                 _ => None,
             }
         };
         let mode = theme_mode
             .or_else(|| env_theme.as_deref().and_then(resolve_string))
             .or_else(|| persisted_theme.as_deref().and_then(resolve_string))
-            .unwrap_or_else(|| {
-                if crate::platform_shell::system_is_dark() {
-                    gpui_component::ThemeMode::Dark
-                } else {
-                    gpui_component::ThemeMode::Light
+            .unwrap_or_else(|| crate::settings::ThemePref::System.resolve(cx));
+        #[cfg(target_os = "linux")]
+        {
+            let configured = env_theme
+                .as_deref()
+                .filter(|value| matches!(*value, "light" | "dark" | "system" | "auto"))
+                .or(persisted_theme.as_deref());
+            cx.set_global(crate::settings::FollowSystemTheme(
+                theme_mode.is_none() && !matches!(configured, Some("light" | "dark")),
+            ));
+            fn sync_system_theme(window: &mut gpui::Window, cx: &mut gpui::App) {
+                if cx
+                    .try_global::<crate::settings::FollowSystemTheme>()
+                    .is_some_and(|follow| follow.0)
+                {
+                    let mode: gpui_component::ThemeMode = window.appearance().into();
+                    if Theme::global(cx).mode != mode {
+                        Theme::change(mode, Some(window), cx);
+                        crate::shell::set_system_theme_pending(matches!(
+                            mode,
+                            gpui_component::ThemeMode::Dark
+                        ));
+                    }
                 }
-            });
+            }
+            cx.observe_new::<gpui_component::Root>(|_, window, cx| {
+                if let Some(window) = window {
+                    sync_system_theme(window, cx);
+                    window.observe_window_appearance(sync_system_theme).detach();
+                }
+            })
+            .detach();
+        }
         Theme::change(mode, None, cx);
         // Sync native chrome (titlebars, traffic lights, menus) to the
         // app theme so secondary windows don't show system-dark chrome

crates/ferail-gpui/src/settings.rs

@@ -122,6 +122,11 @@ pub fn category_from_arg(arg: Option<&str>) -> SettingsCategory {
 // Theme preference: external API
 // =============================================================================
 
+#[cfg(target_os = "linux")]
+pub(crate) struct FollowSystemTheme(pub bool);
+#[cfg(target_os = "linux")]
+impl gpui::Global for FollowSystemTheme {}
+
 #[derive(Clone, Copy, PartialEq, Eq, Debug)]
 pub enum ThemePref {
     Light,
@@ -146,15 +151,22 @@ impl ThemePref {
         }
     }
 
-    fn resolve(self) -> ThemeMode {
+    pub(crate) fn resolve(self, _cx: &App) -> ThemeMode {
         match self {
             ThemePref::Light => ThemeMode::Light,
             ThemePref::Dark => ThemeMode::Dark,
             ThemePref::System => {
-                if crate::platform_shell::system_is_dark() {
-                    ThemeMode::Dark
-                } else {
-                    ThemeMode::Light
+                #[cfg(target_os = "linux")]
+                {
+                    _cx.window_appearance().into()
+                }
+                #[cfg(not(target_os = "linux"))]
+                {
+                    if crate::platform_shell::system_is_dark() {
+                        ThemeMode::Dark
+                    } else {
+                        ThemeMode::Light
+                    }
                 }
             }
         }
@@ -2611,7 +2623,12 @@ fn theme_tile(pref: ThemePref) -> impl IntoElement {
             // when given a Window. `cx.refresh_windows()` propagates
             // the same change to any other open window (e.g. the
             // main Shell while Settings is open in a second window).
-            let resolved = pref.resolve();
+            let resolved = pref.resolve(cx);
+            #[cfg(target_os = "linux")]
+            {
+                cx.set_global(FollowSystemTheme(pref == ThemePref::System));
+                crate::shell::set_system_theme_pending(matches!(resolved, ThemeMode::Dark));
+            }
             Theme::change(resolved, Some(window), cx);
             // Match native chrome across all windows to the new theme.
             crate::platform_shell::set_app_appearance(matches!(