GUISkin.button  

// Modifies only the button style of the current GUISkin

var style : GUIStyle;

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

using UnityEngine;
using System.Collections;

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

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public style as GUIStyle

    def OnGUI():
        GUI.skin.button = style
        GUILayout.Button('This is a button.')