The following group of buttons are "reversible", that is you can customize the face of the button to the alternate symbol (the one shown on top).  When the buttons are enabled (i.e. not "fuzzy"), clicking on them while pressing the Shift key reverses their face:

The  and buttons act in pairs:  if one changes the other changes too.  The other reversible buttons can be changed individually.

For a button to be accessible for change it must be enabled.  However, buttons can be changed anytime via the preferences settings.