[ubuntu] Customizing existing icons

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http://ubuntuforums.org – I'm running Ubuntu 12.04 with gnome 3 shell. I'm currently using the icon theme Ubuntu mono-dark and I like it but there are just a few icons I want to change. I imagine changing those icons would just be going to some directory and replacing the images, so I looked at /usr/share/icons/ubuntu-mono-dark/ but I'm not sure where to go from there. There are 3 folders there, 16, 22, and 24. In 22 and 24, there are icons that say "audio-volume-low-panel.png" "audio-volume-low.png" etc but none of them are the ones that contain the images that appear when I change the volume throu (Hardware)