Replace the lines 454 and 454 in QuickToggle.cs with these two:
- normal = new GUIStyleState() { background = EditorGUIUtility.Load("Icons/d_winbtn_mac_close.png") as Texture2D },
- onNormal = new GUIStyleState() { background = EditorGUIUtility.Load("Icons/d_winbtn_mac_max.png") as Texture2D },