yeah it was really strange. I tried to use every single class available according to documentation and also what you recommend and set all color properties to white like this:
.c3 { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.tick { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-chart { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-chart-line { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-chart-lines { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-chart-bar { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-chart-bars { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-chart-text { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-chart-texts { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-chart-arc { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-chart-arcs { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-chart-arcs-title { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-chart-arcs-background { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-chart-arcs-gauge-unit { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-chart-arcs-gauge-max { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-chart-arcs-gauge-min { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-selected-circle { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-selected-circles { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-event-rect { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-event-rects { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-event-rects-single { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-event-rects-multiple { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-zoom-rect { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-brush { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-focused { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-region { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-regions { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-tooltip { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-tooltip-name { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-shape { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-shapes { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-line { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-lines { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-bar { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-bars { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-circle { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-circles { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-arc { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-arcs { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-area { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-areas { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-empty { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-text { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-texts { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-gauge-value { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-grid { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-xgrid { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-xgrids { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-xgrid-line { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-xgrid-lines { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-xgrid-focus { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-ygrid { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-ygrids { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-ygrid-line { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-ygrid-lines { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-axis { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-axis-x { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-axis-x-label { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-axis-y { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-axis-y-label { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-axis-y2 { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-axis-y2-label { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-legend-item { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-legend-item-event { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-legend-item-tile { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-legend-item-hidden { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
.c3-legend-item-focused { fill: #fff !important; stroke: #fff !important; color: #fff !important; }
Anyway my axis lines are still black :-) So I come up with another idea. I deleted all the previous and add only one rule:
I suddenly my axis are white :-) so I started to try every single svg element and I ended with this:
then I check the svg code and used path class domain to style my axis.