Change ribbon icon depending on light/dark theme

I have a custom plugin which installs a ribbon icon. This works fine with the default dark theme.

However, a user has reported that when they use a light theme, they cannot see the ribbon icon. It turns out that the icon is there, but the grey colour of the icon is invisible with their theme.

What is the best way to manage this? Does the theme give away what the background colour of the ribbon bar is? Or is there a way to change the icon depending on a light or dark theme etc?