Refresh Dark Mode Colours When the System Theme Differs at Startup

On Linux and macOS the app follows the system theme: at startup it
overwrites the saved dark_color_mode with the system appearance. The label
colours and the cached dark mode flag were already set from the saved value
by then, and the later "changed" check reads the mode only after the
overwrite, so it never fires. For that session every dialog that maps its
colours through UpdateDlgDarkUI keeps the old mode, e.g. white dialogs on a
dark main window after switching the desktop to dark, or on a fresh install
on a dark desktop.

Refresh the label colours and the flag when the system theme differs from
the saved mode.

src/slic3r/GUI/GUI_App.cpp

@@ -3232,9 +3232,16 @@ bool GUI_App::on_init_inner()
 #ifdef _MSW_DARK_MODE
 
 #ifndef __WINDOWS__
+    // The label colours and the dark mode flag were set above from the saved mode. When the system theme
+    // differs from it, refresh them, or dialogs that map their colours stay in the old mode for this session.
+    const bool saved_dark_mode = dark_mode();
     wxSystemAppearance app = wxSystemSettings::GetAppearance();
     GUI::wxGetApp().app_config->set("dark_color_mode", app.IsDark() ? "1" : "0");
     GUI::wxGetApp().app_config->save();
+    if (dark_mode() != saved_dark_mode) {
+        init_label_colours();
+        Update_dark_mode_flag();
+    }
 #endif // __APPLE__