GUISkin.horizontalScrollbarThumb  

// Modifies only the horizontal scrollbar thumb of the current GUISkin
var hSbarValue : float;
var style : GUIStyle;

function OnGUI () {
    GUI.skin.horizontalScrollbarThumb = style;
    hSbarValue = GUILayout.HorizontalScrollbar (hSbarValue, 1.0, 0.0, 10.0);
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public float hSbarValue;
    public GUIStyle style;
    void OnGUI() {
        GUI.skin.horizontalScrollbarThumb = style;
        hSbarValue = GUILayout.HorizontalScrollbar(hSbarValue, 1.0F, 0.0F, 10.0F);
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public hSbarValue as single

    public style as GUIStyle

    def OnGUI():
        GUI.skin.horizontalScrollbarThumb = style
        hSbarValue = GUILayout.HorizontalScrollbar(hSbarValue, 1.0F, 0.0F, 10.0F)