I'd like to change the style of the close button on GTK+3 windows with so called "header bars", where the window is rendered without decoration and GTK composes title bar and widgets.

What kind of element is the "X" close button, and what CSS classes does it belong to (if any)?

![Screenshot](https://i.stack.imgur.com/y1CiU.png)

Note that this is *not* part of the window manager theme but actually drawn by GTK+.