After letting Mint do some updates on my mintbox pro this morning, I have the same problem. My Icons looked exactly like yours ie greeny, before and now they look like the strange grey ones.. With the same changed symbols on them.
I also notice my icons for things like file manger have changed (was a small browny icon with a squiggle, now it looks like a file cabinet,) the firefox icon has reverted to the fox and not an eye, the gimp icon has reverted to the head with paintbrush, in firefox the back/forward/reload buttons have changed and so on.
However unlike you restarting the computer does not change things back!
I am running Mint 13 64 bit, running cinnamon --version tells me Cinnamon 1.4.0 (I don't know what it was before the update ran)
My x11 and screen drivers also broke and from having dual screens which were different sizes with one rotated and one normal, I now only have both mirrored and with sharing the same resolution. Un-ticking the Mirror box and clicking apply just throws errors..
I'm totally lost as to how a upgrade should have screwed my system so comprehensively! I've now wasted a whole day trying to fix this but no luck so far.
Thanks for any help!