Example code showing how to use the GUIStyles provided in UnityEditor.EditorStyles class.

GUIStylesExample.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEditor;

public class GUIStylesExample : EditorWindow {

	string textFieldString = "This is a textbox that looks like a bold Label.";

	[MenuItem("My Tools/GUI Styles Example")]
	static void ShowWindow()
	{
		EditorWindow window = GetWindow(typeof(GUIStylesExample));
		window.Show();
	}

	void OnGUI()
	{
		// -----------------------------------------------------------------------
		// GUIStyles applied to GUI controls
		// -----------------------------------------------------------------------
		EditorGUILayout.LabelField("Mini Label", EditorStyles.miniLabel);
		EditorGUILayout.LabelField("Large Label", EditorStyles.largeLabel);
		EditorGUILayout.LabelField("Bold Label", EditorStyles.boldLabel);

		if(GUILayout.Button("Regular Button"))
			Debug.Log("Regular Button pressed.");
		if(GUILayout.Button("Mini Button",EditorStyles.miniButton))
			Debug.Log("Mini Button pressed.");
		if(GUILayout.Button("Toolbar Button",EditorStyles.toolbarButton))
			Debug.Log("Toolbar Button pressed.");
		// -----------------------------------------------------------------------

		// -----------------------------------------------------------------------
		// GUIStyles a certain GUI control applied to another GUI control
		// -----------------------------------------------------------------------
		
		// TextField with Bold Label GUI Style
		textFieldString = EditorGUILayout.TextField(textFieldString, EditorStyles.boldLabel);

		// Button with ColorField GUI Style
		if(GUILayout.Button("ColorField Button",EditorStyles.colorField))
			Debug.Log("ColorField Button.");

		// Button with Centered Grey Mini Label Button GUI Style
		if(GUILayout.Button("Centered Grey Mini Label Button",EditorStyles.centeredGreyMiniLabel))
			Debug.Log("Centered Grey Mini Label Button.");
		// -----------------------------------------------------------------------
	}
}