Where are the iCloud Drive Libraries icons located?
Ok, I found it. iCloud daemon (bird) stores info of all the iCloud containers in:

~/Library/Application Support/CloudDocs/session/containers


The info is stored in a plist file with the name of the container for example:

iCloud.com.pvieito.Example.plist


And the icons are stored in a folder with the name of the container:

iCloud.com.pvieito.Example/40x40_iOS.png
iCloud.com.pvieito.Example/80x80_iOS.png
iCloud.com.pvieito.Example/120x120_iOS.png