The background image used by GUI elements in this given state.

	// Assigns a texture to customStyles[0] for when the control
	// is pressed down
	var aTexture : Texture2D;
	function OnGUI() {
	 	if(!aTexture) {
			Debug.LogError("Assign a texture on the editor first");
			return;
		}
		if(GUI.skin.customStyles.Length > 0)
			GUI.skin.customStyles[0].active.background = aTexture;		
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    public Texture2D aTexture;
    void OnGUI() {
        if (!aTexture) {
            Debug.LogError("Assign a texture on the editor first");
            return;
        }
        if (GUI.skin.customStyles.Length > 0)
            GUI.skin.customStyles[0].active.background = aTexture;
        
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	public aTexture as Texture2D

	def OnGUI() as void:
		if not aTexture:
			Debug.LogError('Assign a texture on the editor first')
			return
		if GUI.skin.customStyles.Length > 0:
			GUI.skin.customStyles[0].active.background = aTexture