Demonstrates how to copy and import Unity's standard UI sprites into a project. This can be useful if the builtin sprites need to be modified or placed onto an atlas.

ExportBuiltinUISprites

using System.Collections.Generic;
using System.IO;
using UnityEditor;
using UnityEngine;
using UnityEngine.SceneManagement;
using UnityEngine.UI;

public static class ExportBuiltinUISprites
{
	private const string outputDirectory = "Assets/Graphics/";

	[MenuItem("Tools/Export Builtin UI Sprites")]
	public static void Export()
	{
		List<SpriteImportInfo> importedAssets;
		try
		{
			AssetDatabase.StartAssetEditing();
			importedAssets = CreateTextureAssets();
		}
		finally
		{
			AssetDatabase.StopAssetEditing();
		}

		if (importedAssets == null)
			return;

		try
		{
			AssetDatabase.StartAssetEditing();
			ApplySpriteImportSettings(importedAssets);
		}
		finally
		{
			AssetDatabase.StopAssetEditing();
		}
	}

	private static IEnumerable<string> StandardResources()
	{
		yield return "UI/Skin/UISprite.psd";
		yield return "UI/Skin/Background.psd";
		yield return "UI/Skin/InputFieldBackground.psd";
		yield return "UI/Skin/Knob.psd";
		yield return "UI/Skin/Checkmark.psd";
		yield return "UI/Skin/DropdownArrow.psd";
		yield return "UI/Skin/UIMask.psd";
	}

	private static List<SpriteImportInfo> CreateTextureAssets()
	{
		var assetInfo = new List<SpriteImportInfo>();

		foreach (var path in StandardResources())
		{
			Sprite sprite = AssetDatabase.GetBuiltinExtraResource<Sprite>(path);
			string outputPath = ExportBuiltinUISprites.outputDirectory + sprite.name + ".png";
			assetInfo.Add(new SpriteImportInfo(sprite, outputPath));

			var texture = CreateReadableCopy(sprite.texture);
			var bytes = texture.EncodeToPNG();
			File.WriteAllBytes(outputPath, bytes);
			AssetDatabase.ImportAsset(
				outputPath,
				ImportAssetOptions.ForceUpdate | ImportAssetOptions.ForceSynchronousImport);
		}
		return assetInfo;
	}

	private static Texture2D CreateReadableCopy(Texture2D texture)
	{
		var bytes = texture.GetRawTextureData();
		var copy = new Texture2D(texture.width, texture.height);
		copy.LoadRawTextureData(bytes);
		return copy;
	}

	private static void ApplySpriteImportSettings(List<SpriteImportInfo> importedAssets)
	{
		for (var i = 0; i < importedAssets.Count; i++)
		{
			Sprite sprite = importedAssets[i].Sprite;
			var importer = AssetImporter.GetAtPath(importedAssets[i].Path);
			var textureImporter = importer as TextureImporter;

			textureImporter.textureCompression = TextureImporterCompression.Uncompressed;
			textureImporter.spriteBorder = sprite.border;
			textureImporter.spritePixelsPerUnit = sprite.pixelsPerUnit;
			textureImporter.spritePivot = sprite.pivot;
			textureImporter.spriteImportMode = SpriteImportMode.Multiple;
			textureImporter.spritesheet = new[]
			{
				new SpriteMetaData
				{
					rect = sprite.rect, name = sprite.name, border = sprite.border
				}
			};

			var settings = new TextureImporterSettings();
			textureImporter.ReadTextureSettings(settings);
			settings.spriteMeshType = SpriteMeshType.FullRect;
			settings.spriteGenerateFallbackPhysicsShape = false;
			textureImporter.SetTextureSettings(settings);

			textureImporter.SaveAndReimport();
		}
	}

	private readonly struct SpriteImportInfo
	{
		public readonly Sprite Sprite;
		public readonly string Path;

		public SpriteImportInfo(in Sprite sprite, in string path)
		{
			this.Sprite = sprite;
			this.Path = path;
		}
	}

	[MenuItem("Tools/Replace builtin sprites in scene")]
	public static void ReplaceInScene()
	{
		var scene = SceneManager.GetActiveScene();
		var roots = scene.GetRootGameObjects();
		foreach (GameObject root in roots)
		{
			foreach (var image in root.GetComponentsInChildren<Image>(includeInactive: true))
			{
				if (image.sprite == null)
					continue;

				string name = image.sprite.name;
				var sprite = AssetDatabase.LoadAssetAtPath<Sprite>(outputDirectory + name + ".png");
				if (sprite != null)
				{
					Undo.RecordObject(image, "Replace Sprite");
					image.sprite = sprite;
					Debug.Log("Replaced: " + image.name, image);
				}
				else
				{
					Debug.Log("Not found: " + image.name, image);
				}
			}
		}
	}
}