Changing Button sprite using OnGui method

Hey! So I have a code which reates a pause menu but want to change the button look so that it isnt that blank transparent thing. How am I supposed to do this?

Here is my code:

 using UnityEngine;
 using System.Collections;
 
 public class PauseMenu : MonoBehaviour
 {
 
     public GUISkin skin;
 
     private float gldepth = -0.5f;
     private float startTime = 0.1f;
 
     public Material mat;
 
     private long tris = 0;
     private long verts = 0;
     private float savedTimeScale;
 
     private bool showfps;
     private bool showtris;
     private bool showvtx;
     private bool showfpsgraph;
 
     public Color lowFPSColor = Color.red;
     public Color highFPSColor = Color.green;
 
     public int lowFPS = 30;
     public int highFPS = 50;
 
     public GameObject start;
 
     public string url = "unity.html";
 
     public Color statColor = Color.yellow;
 
     public string[] credits= {
         "A Fugu Games Production",
         "Programming by Phil Chu",
         "Fugu logo by Shane Nakamura Designs",
         "Copyright (c) 2007-2008 Technicat, LLC"} ;
     public Texture[] crediticons;
 
     public enum Page {
         None,Main,Options,Credits
     }
 
     private Page currentPage;
 
     private float[] fpsarray;
     private float fps;
 
     private int toolbarInt = 0;
     private string[]  toolbarstrings =  {"Audio","Graphics","Stats","System"};
 
 
     void Start() {
         fpsarray = new float[Screen.width];
         Time.timeScale = 1;
         //Comment the next line out if you don't want it to pause right at the beginning
         //PauseGame();
     }
 
     void OnPostRender() {
         if (showfpsgraph && mat != null) {
             GL.PushMatrix ();
             GL.LoadPixelMatrix();
             for (var i = 0; i < mat.passCount; ++i)
             {
                 mat.SetPass(i);
                 GL.Begin( GL.LINES );
                 for (int x=0; x < fpsarray.Length; ++x) {
                     GL.Vertex3(x, fpsarray[x], gldepth);
                 }
                 GL.End();
             }
             GL.PopMatrix();
             ScrollFPS();
         }
     }
 
     void ScrollFPS() {
         for (int x = 1; x < fpsarray.Length; ++x) {
             fpsarray[x-1]=fpsarray[x];
         }
         if (fps < 1000) {
             fpsarray[fpsarray.Length - 1]=fps;
         }
     }
 
     static bool IsDashboard() {
         return Application.platform == RuntimePlatform.OSXDashboardPlayer;
     }
 
     static bool IsBrowser() {
         return (Application.platform == RuntimePlatform.WindowsWebPlayer ||
             Application.platform == RuntimePlatform.OSXWebPlayer);
     }
 
     void LateUpdate () {
         if (showfps || showfpsgraph) {
             FPSUpdate();
         }
 
         if (Input.GetKeyDown("escape")) 
         {
             switch (currentPage) 
             {
             case Page.None: 
                 PauseGame(); 
                 break;
 
             case Page.Main: 
                 if (!IsBeginning()) 
                     UnPauseGame(); 
                 break;
 
             default: 
                 currentPage = Page.Main;
                 break;
             }
         }
     }
 
     void OnGUI () {
         if (skin != null) {
             GUI.skin = skin;
         }
         ShowStatNums();
         ShowLegal();
         if (IsGamePaused()) {
             GUI.color = statColor;
             switch (currentPage) {
             case Page.Main: MainPauseMenu(); break;
             case Page.Options: ShowToolbar(); break;
             case Page.Credits: ShowCredits(); break;
             }
         }   
     }
 
     void ShowLegal() {
         if (!IsLegal()) {
             GUI.Label(new Rect(Screen.width-100,Screen.height-20,90,20),
                 "jdonavan.com");
         }
     }
 
     bool IsLegal() {
         return !IsBrowser() || 
             Application.absoluteURL.StartsWith("http://www.jdonavan.com/") ||
             Application.absoluteURL.StartsWith("http://jdonavan.com/");
 
     }
 
     void ShowToolbar() {
         BeginPage(300,300);
         toolbarInt = GUILayout.Toolbar (toolbarInt, toolbarstrings);
         switch (toolbarInt) {
         case 0: VolumeControl(); break;
         case 3: ShowDevice(); break;
         case 1: Qualities(); QualityControl(); break;
         case 2: StatControl(); break;
         }
         EndPage();
     }
 
     void ShowCredits() {
         BeginPage(300,300);
         foreach(string credit in credits) {
             GUILayout.Label(credit);
         }
         foreach( Texture credit in crediticons) {
             GUILayout.Label(credit);
         }
         EndPage();
     }
 
     void ShowBackButton() {
         if (GUI.Button(new Rect(20, Screen.height - 50, 50, 20),"Back")) {
             currentPage = Page.Main;
         }
     }
 
     void ShowDevice() {
         GUILayout.Label("Unity player version "+Application.unityVersion);
         GUILayout.Label("Graphics: "+SystemInfo.graphicsDeviceName+" "+
             SystemInfo.graphicsMemorySize+"MB\n"+
             SystemInfo.graphicsDeviceVersion+"\n"+
             SystemInfo.graphicsDeviceVendor);
         GUILayout.Label("Shadows: "+SystemInfo.supportsShadows);
         GUILayout.Label("Image Effects: "+SystemInfo.supportsImageEffects);
         GUILayout.Label("Render Textures: "+SystemInfo.supportsRenderTextures);
     }
 
     void Qualities() {
         switch (QualitySettings.GetQualityLevel()) 
         {
         case 0:
             GUILayout.Label("Fastest");
             break;
         case 1:
             GUILayout.Label("Fast");
             break;
         case 2:
             GUILayout.Label("Simple");
             break;
         case 3:
             GUILayout.Label("Good");
             break;
         case 4:
             GUILayout.Label("Beautiful");
             break;
         default:
             GUILayout.Label("Fantastic");
             break;
         }
     }
 
     void QualityControl() {
         GUILayout.BeginHorizontal();
         if (GUILayout.Button("Decrease")) {
             QualitySettings.DecreaseLevel();
         }
         if (GUILayout.Button("Increase")) {
             QualitySettings.IncreaseLevel();
         }
         GUILayout.EndHorizontal();
     }
 
     void VolumeControl() {
         GUILayout.Label("Volume");
         AudioListener.volume = GUILayout.HorizontalSlider(AudioListener.volume, 0, 1);
     }
 
     void StatControl() {
         GUILayout.BeginHorizontal();
         showfps = GUILayout.Toggle(showfps,"FPS");
         showtris = GUILayout.Toggle(showtris,"Triangles");
         showvtx = GUILayout.Toggle(showvtx,"Vertices");
         showfpsgraph = GUILayout.Toggle(showfpsgraph,"FPS Graph");
         GUILayout.EndHorizontal();
     }
 
     void FPSUpdate() {
         float delta = Time.smoothDeltaTime;
         if (!IsGamePaused() && delta !=0.0) {
             fps = 1 / delta;
         }
     }
 
     void ShowStatNums() {
         GUILayout.BeginArea( new Rect(Screen.width - 100, 10, 100, 200));
         if (showfps) {
             string fpsstring= fps.ToString ("#,##0 fps");
             GUI.color = Color.Lerp(lowFPSColor, highFPSColor,(fps-lowFPS)/(highFPS-lowFPS));
             GUILayout.Label (fpsstring);
         }
         if (showtris || showvtx) {
             GetObjectStats();
             GUI.color = statColor;
         }
         if (showtris) {
             GUILayout.Label (tris+"tri");
         }
         if (showvtx) {
             GUILayout.Label (verts+"vtx");
         }
         GUILayout.EndArea();
     }
 
     void BeginPage(int width, int height) {
         GUILayout.BeginArea( new Rect((Screen.width - width) / 2, (Screen.height - height) / 2, width, height));
     }
 
     void EndPage() {
         GUILayout.EndArea();
         if (currentPage != Page.Main) {
             ShowBackButton();
         }
     }
 
     bool IsBeginning() {
         return (Time.time < startTime);
     }
 
 
     void MainPauseMenu() {
         BeginPage(200,200);
         if (GUILayout.Button (IsBeginning() ? "Play" : "Continue")) {
             UnPauseGame();
 
         }
         if (GUILayout.Button ("Options")) {
             currentPage = Page.Options;
         }
         if (GUILayout.Button ("Credits")) {
             currentPage = Page.Credits;
         }
         if (IsBrowser() && !IsBeginning() && GUILayout.Button ("Restart")) {
             Application.OpenURL(url);
         }
         EndPage();
     }
 
     void GetObjectStats() {
         verts = 0;
         tris = 0;
         GameObject[] ob = FindObjectsOfType(typeof(GameObject)) as GameObject[];
         foreach (GameObject obj in ob) {
             GetObjectStats(obj);
         }
     }
 
     void GetObjectStats(GameObject obj) {
         Component[] filters;
         filters = obj.GetComponentsInChildren<MeshFilter>();
         foreach( MeshFilter f  in filters )
         {
             tris += f.sharedMesh.triangles.Length/3;
             verts += f.sharedMesh.vertexCount;
         }
     }
 
     void PauseGame() {
         savedTimeScale = Time.timeScale;
         Time.timeScale = 0;
         AudioListener.pause = true;
 
         currentPage = Page.Main;
     }
 
     void UnPauseGame() {
         Time.timeScale = savedTimeScale;
         AudioListener.pause = false;
         currentPage = Page.None;
 
         if (IsBeginning() && start != null) {
             start.SetActive(true);
         }
     }
 
     bool IsGamePaused() {
         return (Time.timeScale == 0);
     }
 
     void OnApplicationPause(bool pause) {
         if (IsGamePaused()) {
             AudioListener.pause = true;
         }
     }
 }