I try removing the Fullcalendar Buttons borders.
.fc-button { border: 0px !important; }
But when I click on it a new border appear :

And didn't found which element display this border.
I try removing the Fullcalendar Buttons borders.
.fc-button { border: 0px !important; }
But when I click on it a new border appear :

And didn't found which element display this border.
I think that it comes from the box-shadow property that is added to the button thanks to the :focus pseudo-class.
You can try adding this rule :
.fc-button {
box-shadow: none !important;
}