How to remove the "default button" border?

When you have a GtkButton in GTK and it开发者_开发问答 is the default for some window (will be activated when you press Enter), a tiny border is draw around it to inform the user that the button is the default button. How to remove this border ?

I already tried: default-border and default-outside-border style properties.

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After hours finding for this, I've discovered it:

 gtk_button_set_relief(GTK_BUTTON(button_widget), GTK_RELIEF_HALF);

GTK should have a gallery showing how this changes the button behavior.