frames: Don't set prefer-dark-theme if adwaita is loaded

Avoids the following warning in the logs
mutter-x11-frame[4003]: Using
GtkSettings:gtk-application-prefer-dark-theme with libadwaita is
unsupported. Please use AdwStyleManager:color-scheme instead.

When initializing libadwaita, the AdwStyleManager default instance is
initialized taking care of loading the appropriate stylesheet.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4276>

src/frames/main.c

@@ -19,13 +19,16 @@
 
 #include "config.h"
 
+#include "meta-frames-client.h"
 #include "meta-window-tracker.h"
 
 #include <gdk/x11/gdkx.h>
 #include <glib-unix.h>
 #include <gmodule.h>
 #include <X11/extensions/Xfixes.h>
 
+static gboolean should_monitor_color_scheme = TRUE;
+
 typedef void (* InitFunc) (void);
 
 static gboolean
@@ -63,6 +66,7 @@ load_libadwaita (void)
   if (!g_module_symbol (libadwaita, "adw_init", (gpointer *) &adw_init))
     return;
 
+  should_monitor_color_scheme = FALSE;
   adw_init ();
 }
 
@@ -74,6 +78,12 @@ on_sigterm (gpointer user_data)
   return G_SOURCE_REMOVE;
 }
 
+gboolean
+meta_frames_client_should_monitor_color_scheme (void)
+{
+  return should_monitor_color_scheme;
+}
+
 int
 main (int   argc,
       char *argv[])

src/frames/meta-frames-client.h

@@ -0,0 +1,22 @@
+/*
+ * Copyright (C) 2025 Red Hat Inc.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include <glib.h>
+
+gboolean meta_frames_client_should_monitor_color_scheme (void);

src/frames/meta-window-tracker.c

@@ -22,6 +22,7 @@
 #include "meta-window-tracker.h"
 
 #include "meta-frame.h"
+#include "meta-frames-client.h"
 
 #include <gdesktop-enums.h>
 #include <gdk/x11/gdkx.h>
@@ -94,6 +95,8 @@ update_color_scheme (MetaWindowTracker *window_tracker)
   GDesktopColorScheme color_scheme;
   gboolean is_dark;
 
+  g_assert (window_tracker->interface_settings != NULL);
+
   color_scheme = g_settings_get_enum (window_tracker->interface_settings,
                                       "color-scheme");
   is_dark = color_scheme == G_DESKTOP_COLOR_SCHEME_PREFER_DARK;
@@ -422,12 +425,15 @@ meta_window_tracker_class_init (MetaWindowTrackerClass *klass)
 static void
 meta_window_tracker_init (MetaWindowTracker *window_tracker)
 {
-  window_tracker->interface_settings = g_settings_new ("org.gnome.desktop.interface");
-  g_signal_connect (window_tracker->interface_settings,
-                    "changed::color-scheme",
-                    G_CALLBACK (on_color_scheme_changed_cb),
-                    window_tracker);
-  update_color_scheme (window_tracker);
+  if (meta_frames_client_should_monitor_color_scheme ())
+    {
+      window_tracker->interface_settings = g_settings_new ("org.gnome.desktop.interface");
+      g_signal_connect (window_tracker->interface_settings,
+                        "changed::color-scheme",
+                        G_CALLBACK (on_color_scheme_changed_cb),
+                        window_tracker);
+      update_color_scheme (window_tracker);
+    }
 
   window_tracker->frames =
     g_hash_table_new_full (NULL, NULL, NULL,