I did many tests and I checked every c3.js CSS class on my original SVG. I found out that some elements add a style="fill-opacity: 0;". When I turned fill-opacity to 1 and a black background appeared. I did some tests on this elements and it worked. Thanks a lot to every of you that took time to answer, you're awesome guys !

Exemple solution :

const gReact = document.querySelector('.c3-event-rects');

gReact.style.opacity = '1';
gReact.style.fillOpacity = '1';
gReact.style.fill = 'white';