A utility script for Unity 3d: A window that takes the built-in GUI skin, and shows all the custom GUI styles inside.

DldUtil_BuiltInSkinBrowser.cs

using UnityEngine;
using UnityEditor;
using System.Collections;
using System.Text.RegularExpressions;
using System.IO;

public class DldUtil_BuiltInSkinBrowser : EditorWindow
{
	[MenuItem("Window/Built-in Skin Browser")]
	static void OpenWindow()
	{
		DldUtil_BuiltInSkinBrowser window = EditorWindow.GetWindow<DldUtil_BuiltInSkinBrowser>(windowTitle);
		window.Focus();
	}

	const string windowTitle = "Built-in Skin Browser";

	GUISkin _nativeInpsectorSkin;

	Vector2 _scrollPosition = Vector2.zero;

	int _viewOffset = 0;

	System.Text.RegularExpressions.Regex regex;
	string _searchPattern = "";


	void GetNativeSkins()
	{
		_nativeInpsectorSkin = EditorGUIUtility.GetBuiltinSkin(EditorSkin.Inspector);
	}

	void OnEnable()
	{
		GetNativeSkins();
	}

	bool _testToggle = false;
	string _testText = "Testing as textfield";


	Texture2D _normalTextureSelected;
	Texture2D _activeTextureSelected;
	Texture2D _onNormalTextureSelected;
	Texture2D _onActiveTextureSelected;

	void DrawTextureAlpha(Vector2 pos, Texture2D t)
	{
		if (t != null)
		{
			EditorGUI.DrawPreviewTexture(new Rect(pos.x, pos.y, t.width, t.height), t);

			EditorGUI.DrawTextureAlpha(new Rect(pos.x + t.width + 1, pos.y, t.width, t.height), t);
		}
	}

	void OnGUI()
	{

		if (_normalTextureSelected != null || _activeTextureSelected != null || _onNormalTextureSelected != null || _onActiveTextureSelected != null)
		{
			const int padding = 5;

			int currentX = padding;
			int overallHeight = 0;


			if (_normalTextureSelected != null)
			{
				DrawTextureAlpha(new Vector2(currentX, padding), _normalTextureSelected);
				currentX += (_normalTextureSelected.width*2) + 1 + padding;

				overallHeight = Mathf.Max(overallHeight, _normalTextureSelected.height);
			}

			if (_activeTextureSelected != null)
			{
				DrawTextureAlpha(new Vector2(currentX, padding), _activeTextureSelected);
				currentX += (_activeTextureSelected.width*2) + 1 + padding;

				overallHeight = Mathf.Max(overallHeight, _activeTextureSelected.height);
			}

			if (_onNormalTextureSelected != null)
			{
				DrawTextureAlpha(new Vector2(currentX, padding), _onNormalTextureSelected);
				currentX += (_onNormalTextureSelected.width*2) + 1 + padding;

				overallHeight = Mathf.Max(overallHeight, _onNormalTextureSelected.height);
			}

			if (_onActiveTextureSelected != null)
			{
				DrawTextureAlpha(new Vector2(currentX, padding), _onActiveTextureSelected);
				currentX += (_onActiveTextureSelected.width*2) + 1 + padding;

				overallHeight = Mathf.Max(overallHeight, _onActiveTextureSelected.height);
			}

			GUILayout.Space(overallHeight + padding + padding);
		}


		_searchPattern = EditorGUILayout.TextField("Search Pattern:", _searchPattern);
		regex = new Regex(string.IsNullOrEmpty(_searchPattern)? ".*" : _searchPattern);


		int lenToShow = Mathf.Min(_viewOffset + 50, _nativeInpsectorSkin.customStyles.Length);
		

		GUILayout.Label(
			string.Format("{0} custom styles in total. Viewing from {1} to {2}.",
				_nativeInpsectorSkin.customStyles.Length,
				_viewOffset, lenToShow)
		);



		GUILayout.BeginHorizontal();
		if (GUILayout.Button("Prev 50") && (_viewOffset - 50 >= 0))
		{
			_viewOffset -= 50;
		}
		if (GUILayout.Button("Next 50") && (_viewOffset + 50 < _nativeInpsectorSkin.customStyles.Length))
		{
			_viewOffset += 50;
		}
		GUILayout.EndHorizontal();





		_scrollPosition = EditorGUILayout.BeginScrollView(_scrollPosition);
		{
			for (int n = _viewOffset; n < lenToShow; ++n)
			{
				GUIStyle customSkinEntry = _nativeInpsectorSkin.customStyles[n];

				if (string.IsNullOrEmpty(customSkinEntry.name) || !regex.IsMatch(customSkinEntry.name))
				{
					continue;
				}

				GUILayout.BeginHorizontal();
				GUILayout.Label(customSkinEntry.name);

				GUILayout.Space(2);
				GUILayout.Box("               ", customSkinEntry);
				GUILayout.Space(2);
				GUILayout.Button("Testing as Button", customSkinEntry);
				GUILayout.Space(2);
				_testToggle = GUILayout.Toggle(_testToggle, "Testing as Toggle", customSkinEntry);
				GUILayout.Space(2);
				_testText = GUILayout.TextField(_testText, customSkinEntry);

				GUILayout.Space(10);


				GUILayout.BeginHorizontal();
				GUILayout.Box(customSkinEntry.normal.background);
				GUILayout.Space(2);
				GUILayout.Box(customSkinEntry.active.background);
				GUILayout.Space(2);
				GUILayout.Box(customSkinEntry.onNormal.background);
				GUILayout.Space(2);
				GUILayout.Box(customSkinEntry.onActive.background);
				GUILayout.EndHorizontal();


				GUILayout.Space(10);


				// since DrawTextureAlpha doesn't have a EditorGUILayout equivalent, we do this ass-backward way
				if (GUILayout.Button("Show Alpha"))
				{
					_normalTextureSelected = customSkinEntry.normal.background;
					_activeTextureSelected = customSkinEntry.active.background;
					_onNormalTextureSelected = customSkinEntry.onNormal.background;
					_onActiveTextureSelected = customSkinEntry.onActive.background;
				}


				// doesn't work since images from built-in skin isn't set to readable
				/*if (GUILayout.Button("Save image") && customSkinEntry.normal.background != null)
				{
					Texture2D normalH = customSkinEntry.normal.background;
					// haven't tested if this works since it's Pro only
					Texture2D normal = Texture2D.CreateExternalTexture(normalH.width, normalH.height, normalH.format, false, false, normalH.GetNativeTexturePtr());
					var imageAsPng = normal.EncodeToPNG();
					//Debug.Log(imageAsPng);
					File.WriteAllBytes(Application.dataPath + "/../Normal-" + normalH.name + ".png", imageAsPng);
				}*/


				GUILayout.EndHorizontal();

				GUILayout.Space(10);
			}
		}
		EditorGUILayout.EndScrollView();
	}
}