c3/docs/css/samples/style_grid.css

.c3-xgrid-line line {
    stroke: blue;
}
.c3-xgrid-line.grid4 line {
    stroke: pink;
}
.c3-xgrid-line.grid4 text {
    fill: pink;
}
.c3-ygrid-line line {
    stroke: red;
}
.c3-ygrid-line.grid800 line {
    stroke: green;
}
.c3-ygrid-line.grid800 text {
    fill: green;
}