Enable using a dark theme when Gnome "dark style" is set

The dark style setting does not cause a dark theme to be used
automatically, so request it explicitly.

See #23764.

(cherry picked from commit 9b9ec141fbcd604aa8bfc83da6f16fc9bf5c32e0)

Co-authored-by: Colin Kinloch

docs/changes.txt

@@ -295,6 +295,7 @@ wxGTK:
 - Fix GSource leak when using wxIdleEvent::RequestMore() (#23364).
 - Allow using libwebkit2gtk-4.1 and libsoup-3.0 (Scott Talbert, #23630).
 - Support Numpad Enter for ending wxListCtrl editing (Scott Talbert, #23762).
+- Improve dark mode detection (Colin Kinloch and Paul Cornett, #23764).
 
 wxMSW:
 

src/gtk/settings.cpp

@@ -183,6 +183,64 @@ static void notify_gtk_font_name(GObject*, GParamSpec*, void*)
 }
 }
 
+static bool UpdatePreferDark(GVariant* value)
+{
+    // 0: No preference, 1: Prefer dark appearance, 2: Prefer light appearance
+    gboolean preferDark = g_variant_get_uint32(value) == 1;
+
+    GtkSettings* settings = gtk_settings_get_default();
+    char* themeName;
+    gboolean preferDarkPrev;
+    g_object_get(settings,
+        "gtk-theme-name", &themeName,
+        "gtk-application-prefer-dark-theme", &preferDarkPrev, NULL);
+
+    // We don't need to enable prefer-dark if the theme is already dark
+    if (strstr(themeName, "-dark") || strstr(themeName, "-Dark"))
+        preferDark = false;
+    g_free(themeName);
+
+    const bool changed = preferDark != preferDarkPrev;
+    if (changed)
+    {
+        g_object_set(settings,
+            "gtk-application-prefer-dark-theme", preferDark, NULL);
+    }
+    return changed;
+}
+
+// "g-signal" from GDBusProxy
+extern "C" {
+static void
+proxy_g_signal(GDBusProxy*, const char*, const char* signal_name, GVariant* parameters, void*)
+{
+    if (strcmp(signal_name, "SettingChanged") != 0)
+        return;
+
+    const char* nameSpace;
+    const char* key;
+    GVariant* value;
+    g_variant_get(parameters, "(&s&sv)", &nameSpace, &key, &value);
+    if (strcmp(nameSpace, "org.freedesktop.appearance") == 0 &&
+        strcmp(key, "color-scheme") == 0)
+    {
+        if (UpdatePreferDark(value))
+        {
+            for (int i = wxSYS_COLOUR_MAX; i--;)
+                gs_systemColorCache[i].UnRef();
+
+            for (auto* win: wxTopLevelWindows)
+            {
+                wxSysColourChangedEvent event;
+                event.SetEventObject(win);
+                win->HandleWindowEvent(event);
+            }
+        }
+    }
+    g_variant_unref(value);
+}
+}
+
 // Some notes on using GtkStyleContext. Style information from a context
 // attached to a non-visible GtkWidget is not accurate. The context has an
 // internal visibility state, controlled by the widget, which it presumably
@@ -1124,12 +1182,62 @@ bool wxSystemSettingsNative::HasFeature(wxSystemFeature index)
 class wxSystemSettingsModule: public wxModule
 {
 public:
-    virtual bool OnInit() wxOVERRIDE { return true; }
+    virtual bool OnInit() wxOVERRIDE;
     virtual void OnExit() wxOVERRIDE;
+
+#ifdef __WXGTK3__
+    GDBusProxy* m_proxy;
+#endif
     wxDECLARE_DYNAMIC_CLASS(wxSystemSettingsModule);
 };
 wxIMPLEMENT_DYNAMIC_CLASS(wxSystemSettingsModule, wxModule);
 
+bool wxSystemSettingsModule::OnInit()
+{
+#ifdef __WXGTK3__
+    // Gnome has gone to a dark style setting rather than a selectable dark
+    // theme, available via GSettings as the 'color-scheme' key under the
+    // 'org.gnome.desktop.interface' schema. It's also available via a "portal"
+    // (https://docs.flatpak.org/en/latest/portal-api-reference.html), which
+    // has the advantage of allowing the setting to be accessed from within a
+    // virtualized environment such as Flatpak. Since the setting does not
+    // change the theme, we propagate it to the GtkSettings
+    // 'gtk-application-prefer-dark-theme' property to get a dark theme.
+
+    m_proxy = NULL;
+
+    // GTK_THEME environment variable overrides other settings
+    if (getenv("GTK_THEME") == NULL)
+    {
+        m_proxy = g_dbus_proxy_new_for_bus_sync(
+            G_BUS_TYPE_SESSION, G_DBUS_PROXY_FLAGS_NONE, NULL,
+            "org.freedesktop.portal.Desktop",
+            "/org/freedesktop/portal/desktop",
+            "org.freedesktop.portal.Settings",
+            NULL, NULL);
+    }
+    if (m_proxy)
+    {
+        g_signal_connect(m_proxy, "g-signal", G_CALLBACK(proxy_g_signal), NULL);
+
+        GVariant* ret = g_dbus_proxy_call_sync(m_proxy, "Read",
+            g_variant_new("(ss)", "org.freedesktop.appearance", "color-scheme"),
+            G_DBUS_CALL_FLAGS_NONE, -1, NULL, NULL);
+        if (ret)
+        {
+            GVariant* child;
+            g_variant_get(ret, "(v)", &child);
+            GVariant* value = g_variant_get_variant(child);
+            UpdatePreferDark(value);
+            g_variant_unref(value);
+            g_variant_unref(child);
+            g_variant_unref(ret);
+        }
+    }
+#endif // __WXGTK3__
+    return true;
+}
+
 void wxSystemSettingsModule::OnExit()
 {
 #ifdef __WXGTK3__
@@ -1141,6 +1249,8 @@ void wxSystemSettingsModule::OnExit()
         g_signal_handlers_disconnect_by_func(settings,
             (void*)notify_gtk_font_name, NULL);
     }
+    if (m_proxy)
+        g_object_unref(m_proxy);
 #endif
     if (gs_tlw_parent)
     {