How to customize guistyle

I have a project using playmaker but I could not figure out where to set the sytle my unity3d editor is version 5.3 and this project is seems to build by 4.3 or before, and now it could run well in 5.3 but I still dont know how it could set a style key as "ui4" to show the customize style.

For layout, it show buttons with png texture,

Any idea? is it a unity3d 4.x feature that could edit customize style?