Do they show up when using a default Metropolis theme?
My understanding of themes is that if an icon is called for by XBMC and it is not in the "theme texture" file, it will look for it in the original textures.xbt file. So if the icons do work in the default Metropolis theme, they should work with mine...
If you are feeling brave, you could do what I did... download the Metropolis 2.6.3 theme from the GIT, extract it to your drive, and modify the images yourself with a graphics program. I use paint.NET (free).
I am posting a "complete" texture file that includes all of the original artwork from Metropolis plus my mods. It can replace the original textures.xbt if you don't want to switch to Glass, Leather...etc. Or, just rename it to Metro or Metrodark and add it to the media folder. PS, I added the original icons back.
new Textures file
Hope this helps.