GUISkin.textField  

// Modifies only the textField style of the current GUISkin

var style : GUIStyle;
private var str : String = "A string...";

function OnGUI() {
    GUI.skin.textField = style;
    str = GUILayout.TextField(str);
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public GUIStyle style;
    private string str = "A string...";
    void OnGUI() {
        GUI.skin.textField = style;
        str = GUILayout.TextField(str);
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public style as GUIStyle

    private str as string = 'A string...'

    def OnGUI():
        GUI.skin.textField = style
        str = GUILayout.TextField(str)