Add option to change icon

My Firefox theme is dark, and it's nearly impossible to see the icon.

Either should it be possible to change the icon, or, like there is a dark/light theme, there should be a dark and a light icon.

See theme_icons here. I think an inverted icon for dark themes should suffice.

Okay, the above seems quite limited and it doesn't work for me locally anyway. Onto the next solution... :thinking: