How to change buttons in menu bar

I am trying to figure out how to add/change the images in the menu bar (ex, change the “close button” to some other picture).

I saw the text field example in the tutorial, but I am hoping there is a more efficient way to add a button to the menu bar.

For anybody else wondering how to find this, look at chapter 6 in the common pdf. The buttons in the menu bar are called “actions”.