Glad to hear that it was a false positive.
But more to the point, yes it's very possible for malware (virus, spyware, adware, etc.) to hide inside a wb skin folder. It also seems that this would be a great way to transmit viruses & spyware to alot of other pc's if people aren't careful.
All of the submissions that are uploaded to this site are personally reviewed by moderators before being released to the downloading public. If some form of malware did get zipped up inside of a wb theme file, I'm sure someone at this site would have found it before it gets released to the rest of us.
However, take responsibility for your own pc. Scan for viruses & spyware regularly and make sure your machine is up to date with the latest & greatest security/critical patches from M$ , and if you're running winxp (or win2k pro?), alot of users will be running as the local admin account for their machine (instead of running as a restricted user account type) - don't download & install every piece of software on this planet. If you do download & install something on your machine, make sure that it's virus/spyware free - don't assume that it's clean and don't download pirated software: the latest fad for malware authors is to hide their spyware & virus creations inside of much sought after software that people download for free from some p2p/torrent site.
The next time you download some form of "free" warez, take into account you may be getting more than just pirated software installed on your machine.