GUISkin.window  

// Modifies only the window style of the current GUISkin

var style : GUIStyle;
var windowRect : Rect = Rect (20, 20, 120, 50);

function OnGUI() {
    GUI.skin.window = style;
    windowRect = GUILayout.Window (0, windowRect, DoMyWindow, "My Window");
}

// Make the contents of the window
function DoMyWindow (windowID : int) {
     // This button will size to fit the window
    if (GUILayout.Button ("Hello World"))
        print ("Got a click");
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public GUIStyle style;
    public Rect windowRect = new Rect(20, 20, 120, 50);
    void OnGUI() {
        GUI.skin.window = style;
        windowRect = GUILayout.Window(0, windowRect, DoMyWindow, "My Window");
    }
    void DoMyWindow(int windowID) {
        if (GUILayout.Button("Hello World"))
            print("Got a click");
        
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public style as GUIStyle

    public windowRect as Rect = Rect(20, 20, 120, 50)

    def OnGUI():
        GUI.skin.window = style
        windowRect = GUILayout.Window(0, windowRect, DoMyWindow, 'My Window')

    def DoMyWindow(windowID as int):
        if GUILayout.Button('Hello World'):
            print('Got a click')