FIX: dark mode 3rd printer bed display

1.fix custom bed render grid inappropriately
2.optimize: do not init toolbar every time switch dark mode

Change-Id: I354496c6c982d9f72b22629f6973a79d65189dda

src/slic3r/GUI/GLCanvas3D.hpp

@@ -1044,6 +1044,7 @@ public:
 private:
     bool _is_shown_on_screen() const;
 
+    void _switch_toolbars_icon_filename();
     bool _init_toolbars();
     bool _init_main_toolbar();
     bool _init_select_plate_toolbar();