.btn-calendar-nav span.fas { padding: 0 0.1rem; }

#calendar-month div, #btn-previous-year, #btn-next-year {
    border: 1px solid white;
}

#calendar-month div:hover, #btn-previous-year:hover, #btn-next-year:hover {
    border: 1px solid blue;
    cursor: pointer;
}