Aha, yeah looks like that’s why the changes weren’t doing anything–there’s no actual <svg> tag. I don’t think it’s a bug that the component put it in an <img> tag. That’s just one of the ways to display an svg, but it does make it harder to style it. If you have access to the svg files themselves, you can change the stroke or fill color there (it would be an attribute like stroke="black", and there could be more than one).
If you can’t edit the svg, you could try using css filters. These let you add certain visual effects to an element. Since they appear to be black now, we could try inverting them to get white.
.custom-header-icon-link img {
filter: invert(100%);
}
Note that the selector needs to be img instead of svg now