I would like to change that, but currently it can not be fixed reliably. The extension already ships separate icons for dark and light themes, but for some reason Firefox does not always pick the correct icon. (I get wrong icon by using a desktop wide dark theme, but selecting a dark theme in Firefox itself results in the correct one being used). Also the colors follow mozilla's guidelines, which might not fit the theme even if the correct icon variant is in use.

If Firefox ever starts to support using colors from the theme itself, i'll switch to that.

Thank you for the comment.