feat(theme): implement Auto mode via system color-scheme

src/app/application.cpp

@@ -330,6 +330,22 @@ void Application::initServices() {
   m_themeService.apply();
   m_configService.addReloadCallback([this]() { m_themeService.onConfigReload(); });
 
+  // Watch the dconf user database so Auto mode reacts immediately to system
+  // color-scheme changes (org.gnome.desktop.interface color-scheme).
+  {
+    const char* xdg = std::getenv("XDG_CONFIG_HOME");
+    const char* home = std::getenv("HOME");
+    std::filesystem::path dconfDb;
+    if (xdg != nullptr && xdg[0] != '\0') {
+      dconfDb = std::filesystem::path(xdg) / "dconf" / "user";
+    } else if (home != nullptr && home[0] != '\0') {
+      dconfDb = std::filesystem::path(home) / ".config" / "dconf" / "user";
+    }
+    if (!dconfDb.empty()) {
+      m_fileWatcher.watch(dconfDb, [this]() { m_themeService.onAutoSchemeChanged(); });
+    }
+  }
+
   if (!m_wayland.connect()) {
     throw std::runtime_error("failed to connect to Wayland display");
   }

src/theme/theme_service.cpp

@@ -17,6 +17,7 @@
 #include <exception>
 #include <filesystem>
 #include <fstream>
+#include <gio/gio.h>
 #include <json.hpp>
 #include <sstream>
 #include <string>
@@ -38,7 +39,37 @@ namespace noctalia::theme {
       std::string mode;
     };
 
-    std::string resolvedModeName(const ThemeConfig& cfg) { return cfg.mode == ThemeMode::Light ? "light" : "dark"; }
+    // Returns "light" or "dark" from org.gnome.desktop.interface color-scheme.
+    std::string_view readSystemColorScheme() {
+      static GSettings* settings = []() -> GSettings* {
+        GSettingsSchemaSource* source = g_settings_schema_source_get_default();
+        if (source == nullptr)
+          return nullptr;
+        GSettingsSchema* schema = g_settings_schema_source_lookup(source, "org.gnome.desktop.interface", TRUE);
+        if (schema == nullptr)
+          return nullptr;
+        const bool hasKey = g_settings_schema_has_key(schema, "color-scheme") != FALSE;
+        g_settings_schema_unref(schema);
+        if (!hasKey)
+          return nullptr;
+        return g_settings_new("org.gnome.desktop.interface");
+      }();
+
+      if (settings == nullptr)
+        return "dark";
+      gchar* raw = g_settings_get_string(settings, "color-scheme");
+      if (raw == nullptr)
+        return "dark";
+      const bool isLight = (std::string_view(raw) == "prefer-light");
+      g_free(raw);
+      return isLight ? "light" : "dark";
+    }
+
+    std::string resolvedModeName(const ThemeConfig& cfg) {
+      if (cfg.mode == ThemeMode::Auto)
+        return std::string(readSystemColorScheme());
+      return cfg.mode == ThemeMode::Light ? "light" : "dark";
+    }
 
     ResolvedTheme resolveBuiltin(const ThemeConfig& cfg) {
       const auto* palette = findBuiltinPalette(cfg.builtinPalette);
@@ -240,6 +271,12 @@ namespace noctalia::theme {
     }
   }
 
+  void ThemeService::onAutoSchemeChanged() {
+    if (m_config.config().theme.mode == ThemeMode::Auto) {
+      resolveAndSet(/*animate=*/true);
+    }
+  }
+
   void ThemeService::toggleLightDark() {
     const auto next = m_isLightMode ? ThemeMode::Dark : ThemeMode::Light;
     // Persist via ConfigService → StateService. The resulting overrides-change

src/theme/theme_service.h

@@ -31,6 +31,7 @@ namespace noctalia::theme {
     // Resolves the target theme and cross-fades to it.
     void onConfigReload();
     void onWallpaperChange();
+    void onAutoSchemeChanged();
     void toggleLightDark();
     void cycleMode();
     [[nodiscard]] ThemeMode configuredMode() const noexcept;