I think this occured after doing an 'apt-get update' and then 'apt-get upgrade' which I thought would keep elementary OS among other applications, up to date. After a restart, I was presented with fugly Ubuntu icons and generic application icons. Any idea whatsoever how I can revert back to elementary OS' icons? I'm sure they're available, I just don't get why the Ubuntu theme replaced these icons. Did the 'apt-get upgrade' somehow think that I wanted the latest of the Ubuntu environment? Both the Wing panel, Dock among icons for Photos, Calendar, Mail and so on were affected.
Note that no other applications were installed through the upgrade, only the useless change of theme.