public GUISkin custom_skin; void OnGUI() { GUI.skin = custom_skin; GUI.Label(new Rect(10, 10, 100, 20), "Hello World"); if(GUI.Button(new Rect(10, 40, 100, 20), "Click me")) { Debug.Log("Button clicked"); } }This code sets the GUI skin to a custom skin and applies it to all the GUI elements rendered in the OnGUI function. The package library for GUISkin is the UnityEngine module provided by the Unity engine.