.myCssClass{
    font-size:20px;
}

/* cor da pie e donut*/
g.c3-chart-arcs text {
    fill: black !important;
    font-size: 10px !important;
}
/* cor da bar*/
.c3-chart-bars ~ .c3-chart-texts text.c3-text {
    fill: black !important;
    font-size: 10px;
}