I can see how to set the styles for an “enabled” button, but not how to tell GUI.Button whether the button is “enabled” (or has keyboard focus) or not.
Enabled is set like this:
GUI.enabled = false;
GUI.Button(...)
But when a control has keyboard focus, it uses the active style.
This control is the focused control, which you can find by GUI.GetNameOfFocusedControl.