[Linux] revise dark theme detection

shell/platform/linux/fl_settings_plugin.cc

@@ -4,6 +4,8 @@
 
 #include "flutter/shell/platform/linux/fl_settings_plugin.h"
 
+#include <gmodule.h>
+#include <gtk/gtk.h>
 #include <cstring>
 
 #include "flutter/shell/platform/linux/public/flutter_linux/fl_basic_message_channel.h"
@@ -17,7 +19,6 @@ static constexpr char kPlatformBrightnessLight[] = "light";
 static constexpr char kPlatformBrightnessDark[] = "dark";
 
 static constexpr char kDesktopInterfaceSchema[] = "org.gnome.desktop.interface";
-static constexpr char kDesktopGtkThemeKey[] = "gtk-theme";
 static constexpr char kDesktopTextScalingFactorKey[] = "text-scaling-factor";
 static constexpr char kDesktopClockFormatKey[] = "clock-format";
 static constexpr char kClockFormat24Hour[] = "24h";
@@ -32,6 +33,27 @@ struct _FlSettingsPlugin {
 
 G_DEFINE_TYPE(FlSettingsPlugin, fl_settings_plugin, G_TYPE_OBJECT)
 
+static bool is_dark_color(GdkRGBA* color) {
+  return color->red < 0.5 && color->green < 0.5 && color->blue < 0.5;
+}
+
+static bool is_dark_theme() {
+  // GTK doesn't have a specific flag for dark themes, so we check if the
+  // style text color is light or dark
+  GList* windows = gtk_window_list_toplevels();
+  if (windows == nullptr) {
+    return false;
+  }
+
+  GtkWidget* window = GTK_WIDGET(windows->data);
+  g_list_free(windows);
+
+  GdkRGBA color;
+  GtkStyleContext* style = gtk_widget_get_style_context(window);
+  gtk_style_context_get_color(style, GTK_STATE_FLAG_NORMAL, &color);
+  return !is_dark_color(&color);
+}
+
 // Sends the current settings to the Flutter engine.
 static void update_settings(FlSettingsPlugin* self) {
   gdouble scaling_factor = 1.0;
@@ -44,15 +66,10 @@ static void update_settings(FlSettingsPlugin* self) {
     g_autofree gchar* clock_format =
         g_settings_get_string(self->interface_settings, kDesktopClockFormatKey);
     always_use_24hr = g_strcmp0(clock_format, kClockFormat24Hour) == 0;
+  }
 
-    // GTK doesn't have a specific flag for dark themes, so we have some
-    // hard-coded themes for Ubuntu (Yaru) and GNOME (Adwaita).
-    g_autofree gchar* gtk_theme =
-        g_settings_get_string(self->interface_settings, kDesktopGtkThemeKey);
-    if (g_strcmp0(gtk_theme, "Yaru-dark") == 0 ||
-        g_strcmp0(gtk_theme, "Adwaita-dark") == 0) {
-      platform_brightness = kPlatformBrightnessDark;
-    }
+  if (is_dark_theme()) {
+    platform_brightness = kPlatformBrightnessDark;
   }
 
   g_autoptr(FlValue) message = fl_value_new_map();