i've added a lot of icon to my custom icon theme. I love the Rosa os 2012 artwork: [link]
I've took a few of its icon set and from other sources.
the icons on plank in order are: file manager, chrome, geary, shotwell, scratch, empathy, noise, software center, ubuntu tweak, switchboard, stacks...., and dropoff
i also change some icons in the top panel
a version of my custom icon set can be found in my gallery it's names Leon