// Modifies only the box style of the current GUISkin

var style : GUIStyle;

function OnGUI() {
    GUI.skin.box = style;
    GUILayout.Box("This is a box.");
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public GUIStyle style;
    void OnGUI() {
        GUI.skin.box = style;
        GUILayout.Box("This is a box.");
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public style as GUIStyle

    def OnGUI():
        GUI.skin.box = style
        GUILayout.Box('This is a box.')