GUILayout.BeginHorizontal with a visual border/background?

I'm using GUILayout.BeginHorizontal() to create a series of panes (there are many other nested horizontal/vertical areas). I need a visible border or background image to cover the entire width/height of each of these horizontal/vertical areas so that the user can tell which parts represent sections. I don't see an option in my GUISkin to do this. Anyone know how I can do this?

GUILayout.BeginHorizontal() has an optional GUIStyle argument. Pass in a style there (e.g. "Box") and it will use it.

(doc reference)

Here is a copy and paste solution with minimal code. Kudos to @Molix for providing the means to this answer.

GUILayout.BeginHorizontal(EditorStyles.helpBox);
// Your wrapped code here
GUILayout.EndHorizontal();

Or you can check this out. It has something about horizontal and vertical layouts in a Custom Inspector.
http://answers.unity3d.com/answers/1288008/view.html