I recently switched to gnome 3 (love it!), but the default icon theme was a little dreary and gray, so I switched that back to ubuntu-mono-dark. But now whenever I change my volume, I get this ugly pixelated icon show up. The forum says I don't have enough reputation to post an screenshot, but it's pixelated and ugly with these fuzzy straight blue lines that turn on as the volume goes up.

I found identical images in the Humanity and Humanity-Dark icon themes (one example: /usr/share/icons/Humanity/status/24/audio-volume-high.png). I tinkered with the images by changing their names, temporarily deleting them, etc. But it had no effect on the ugly icon. What file is responsible for violating the beauty of my desktop?! Once I find it, I can replace it.