In all cases the SVG needs to have a single color icon (black works for me, not sure about other colors) with transparent background.
I use Affinity Designer (which I find quite powerful and handy), and there are two steps to solving it. I'm guessing the same Rivet that helped me solve it on the Affinity Forums is the one who answered here:
Someone also replied below that with info on how to use Sketch. Not sure if these answers apply 1-to-1 with whatever graphics software you use, but might be worth checking out.
There's also this converter: Didn't end up working for me, but might work for you FMSVGInator | Proof+Geist