pillmuncher Until this is fixed upstream, a workaround of this is to delete system-shutdown-symbolic.svg
file and let it to be inherited by system. In this case I recommend you to use the theme from source https://github.com/numixproject/numix-icon-theme
and not installing it from Solus repository because it will result as a broken package after you have deleted system-shutdown-symbolic.svg
file.
I don't know if there are other icons in this theme with same problem, but I am assuming that shutdown icon is the only one. I will keep an eye to numix project if they do a new release and then I will update it.