oh forgot to mention, S60 themes do not change your sounds, well not yet but recent devices look like they might be going that way. Viruses are a non issue, the included anti-virus creates way more trouble by creating striking fear into new owners who don't know better. Now thats not to say that its not possible, of course a virus might be released at some point in time, but in the many years that S60 has been around i don't know of anything more than a proof of concept being made. Regarding bad/malicious themes sounds aren't linked in on S60 themes yet. I've never run across a theme that rendered my device unusable. Some of them do have some bad color schemes that in certain situations give no contrast but those are few and far between and its only usually on a rather unimportant dialog. I guess you should just exercise judgement in both what the theme looks like as well as the trustworthiness of the source.
One feature to protect you is that the developers digitally sign their applications. Themes and applications have to be signed for your phone to be able to install them. This can be cumbersome for developers but its done to protect us as the end users.
