How to add a GUI skin to this script!

Hello, Could someone make it so that this script has a variable GUI skin slot! using UnityEngine; using System.Collections;

 [System.Serializable]
 public class Map
 {
     public string name;
     public string levelName;
 }
  
 public class MainMenu : MonoBehaviour
 {
     public int curMenu = 0;
  
     private string[] settings;
     private int curIndex;
  
     public float dif = 1;
     HostData[] datas;
     public Vector2 scroll;
     private string gameName = "Server 666";
  
     //You have to change this!
     public string uniqueGameName = "MyUniqueGameName";
  
     //After which time will the server list be refreshed ?
     public float refreshTime = 10;
  
     public Map[] maps;
     public int curMapIndex;
  
     void Start()
     {
         InvokeRepeating("GetHostList", 0, refreshTime);
         settings = QualitySettings.names;
         Network.isMessageQueueRunning = false;
     }
  
     void GetHostList()
     {
         MasterServer.RequestHostList(uniqueGameName);
     }
  
     void Update()
     {
         dif = (Screen.width / 12.8f) / 100;
         datas = MasterServer.PollHostList();
     }
  
     void OnGUI()
     {
         if (curMenu == 0)
         {
             if (GUI.Button(new Rect(Screen.width / 2 - 100 * dif, Screen.height / 2 - 80 * dif, 200 * dif, 50 * dif), "Play"))
             {
                 curMenu = 1;
             }
             if (GUI.Button(new Rect(Screen.width / 2 - 100 * dif, Screen.height / 2 - 25 * dif, 200 * dif, 50 * dif), "Options"))
             {
                 curMenu = 2;
             }
             if (GUI.Button(new Rect(Screen.width / 2 - 100 * dif, Screen.height / 2 + 30 * dif, 200 * dif, 50 * dif), "Quit"))
             {
                 Application.Quit();
             }
  
         }
         else if (curMenu == 1)
         {
             GUI.Box(new Rect(20 * dif, 20 * dif, 200 * dif, 200 * dif), "");
             GUILayout.BeginArea(new Rect(25 * dif, 25 * dif, 190 * dif, 190 * dif));
             gameName = GUILayout.TextField(gameName);
             if (GUILayout.Button("Start Server"))
             {
                 Network.InitializeSecurity();
                 Network.InitializeServer(17, 25001, /*!Network.HavePublicAddress()*/ true);
                 MasterServer.RegisterHost(uniqueGameName, gameName);
             }
             if (GUILayout.Button("Select Map"))
             {
                 curMenu = 5;
             }
             if (GUILayout.Button("Direct Connect"))
             {
                 Network.Connect("127.0.0.1", 25001);
             }
             if (GUILayout.Button("Back"))
             {
                 curMenu = 0;
             }
             GUILayout.EndArea();
             GUI.Box(new Rect(Screen.width - 400 * dif, 0, 400 * dif, Screen.height), "");
             GUILayout.BeginArea(new Rect(Screen.width - 400 * dif, 0, 400 * dif, Screen.height));
             GUILayout.Label("Avaiable Servers: " + datas.Length);
             scroll = GUILayout.BeginScrollView(scroll);
             foreach (HostData data in datas)
             {
                 GUILayout.BeginHorizontal();
                 GUILayout.Label(data.gameName + " Players: " + data.connectedPlayers + " / " + data.playerLimit);
                 if (GUILayout.Button("Connect"))
                 {
                     Network.Connect(data);
                 }
                 GUILayout.EndHorizontal();
             }
             GUILayout.EndScrollView();
             GUILayout.EndArea();
         }
         else if (curMenu == 2)
         {
             if (GUI.Button(new Rect(Screen.width / 2 - 100 * dif, Screen.height / 2 - 80 * dif, 200 * dif, 50 * dif), "Audio"))
             {
                 curMenu = 3;
             }
             if (GUI.Button(new Rect(Screen.width / 2 - 100 * dif, Screen.height / 2 - 25 * dif, 200 * dif, 50 * dif), "Graphics"))
             {
                 curMenu = 4;
             }
             if (GUI.Button(new Rect(Screen.width / 2 - 100 * dif, Screen.height / 2 + 30 * dif, 200 * dif, 50 * dif), "Back"))
             {
                 curMenu = 0;
             }
         }
         else if (curMenu == 3)
         {
             AudioListener.volume = GUI.HorizontalSlider(new Rect(Screen.width / 2 - 100 * dif, Screen.height / 2 - 25 * dif, 200 * dif, 50 * dif), AudioListener.volume, 0f, 1f);
             GUI.Label(new Rect(Screen.width / 2 - 100 * dif, Screen.height / 2 - 45 * dif, 200 * dif, 50 * dif), "Volume: " + (AudioListener.volume * 100).ToString("F0") + " %");
             if (GUI.Button(new Rect(Screen.width / 2 - 100 * dif, Screen.height / 2 + 30 * dif, 200 * dif, 50 * dif), "Back"))
             {
                 curMenu = 2;
             }
         }
         else if (curMenu == 4)
         {
             if (GUI.Button(new Rect(Screen.width / 2 - 75 * dif, Screen.height / 2 - 25 * dif, 150 * dif, 50 * dif), settings[curIndex]))
             {
                 QualitySettings.SetQualityLevel(curIndex);
             }
             if (GUI.Button(new Rect(Screen.width / 2 - 100 * dif, Screen.height / 2 - 25 * dif, 20 * dif, 50 * dif), "←"))
             {
                 if (curIndex != 0) curIndex--;
                 else curIndex = settings.Length - 1;
             }
  
             if (GUI.Button(new Rect(Screen.width / 2 + 80 * dif, Screen.height / 2 - 25 * dif, 20 * dif, 50 * dif), "→"))
             {
                 if (curIndex != settings.Length - 1) curIndex++;
                 else curIndex = 0;
             }
  
             //I only made switching between quality settings. You can make it much more complex if you want.
             if (GUI.Button(new Rect(Screen.width / 2 - 100 * dif, Screen.height / 2 + 30 * dif, 200 * dif, 50 * dif), "Back"))
             {
                 curMenu = 2;
             }
         }
         else if (curMenu == 5)
         {
             if (GUI.Button(new Rect(Screen.width / 2 - 75 * dif, Screen.height / 2 - 25 * dif, 150 * dif, 50 * dif), maps[curMapIndex].name))
             {
             }
             if (GUI.Button(new Rect(Screen.width / 2 - 100 * dif, Screen.height / 2 - 25 * dif, 20 * dif, 50 * dif), "←"))
             {
                 if (curMapIndex != 0) curMapIndex--;
                 else curMapIndex = maps.Length - 1;
             }
  
             if (GUI.Button(new Rect(Screen.width / 2 + 80 * dif, Screen.height / 2 - 25 * dif, 20 * dif, 50 * dif), "→"))
             {
                 if (curMapIndex != maps.Length - 1) curMapIndex++;
                 else curMapIndex = 0;
             }
  
             if (GUI.Button(new Rect(Screen.width / 2 - 100 * dif, Screen.height / 2 + 30 * dif, 200 * dif, 50 * dif), "Back"))
             {
                 curMenu = 1;
             }
         }
     }
  
     void OnServerInitialized()
     {
         networkView.RPC("LoadLevel", RPCMode.AllBuffered, maps[curMapIndex].levelName);
         Debug.Log("Server initialized and ready");
     }
  
     [RPC]
     IEnumerator LoadLevel(string level)
     {
         Network.isMessageQueueRunning = false;
         yield return new WaitForSeconds(1);
         Application.LoadLevel(level);
         Network.isMessageQueueRunning = true;
     }
  
     void OnConnectedToServer()
     {
         Debug.Log("Connected to server");
         //Network.isMessageQueueRunning = true;
     }
 }