The API only allows me to pick one icon for the add-on, so I went with something that worked well on the default theme (which most people use) AND dark themes (which I use). I'm waiting for Mozilla to allow add-ons to have icons follow the user theme's color scheme, but I've yet to see this be allowed officially. Otherwise, while API does allow me to give people the option to pick a color for the icon, I find this is not an elegant solution to this problem.