Comment 1 for bug 1630380

I looked at how XFCE defines it icons.

It is possible to add a status icon for elementary-xfce which is a best fit for light themes. The dark icon set elementary-xfce-dark unfortunately hasn't defined a folder for status icons though.

This means from a 3rd party package it is not possible to set a different icons for elementary-xfce and elementary-xfce-dark status icons.

This makes it impossible for the Diodon package to define an icon which looks good on dark and light themes on XFCE. So I am afraid the only way to set a monochrome icon for XFCE theme would be if the theme/icon maintainer would add it to its package or let's 3rd party packages overwrite the status icon.

As a workaround though you can set a monochrome icon yourself see here: https://esite.ch/2014/07/where-is-my-diodon-monochrome-icon/