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/Gizmos/GLGizmosManager.hpp

@@ -163,6 +163,8 @@ public:
 
     explicit GLGizmosManager(GLCanvas3D& parent);
 
+    void switch_gizmos_icon_filename();
+
     bool init();
 
     bool init_icon_textures();