You can open a issue on github if you want to, but from what you've written here:

If you're using icon mode: white text and transparent background seem to work for me

If you're using badge mode: this mode was more of an afterthought - there seem to be better addons out there for this case. No badge background isn't possible (though I could implement a transparent icon underneath) - the API only allows opaque colors. Setting the badge text color separately has also only been supported since Firefox 63

maybe you could clarify what exactly you're trying to do which isn't working