First of all, WTF is "Symbian Signing"?

Second, is there any way to deconstruct or reverse engineer the theme's sis packages, i.e. be able to extract icons etc. from them.
@Symbian Signing
It's about security issues as told by Cool G5. I mean program before being installed in your s60v3 must be passed & signed for compability as well as security issues. If not signed, program is useless.
@Decomplier
There is no software till date that would extract icons or graphics from theme installer package. There are some software (you may google), which claims they can do it but I don't believe as I have already tried three progs.
Why do you need this? Just create your own icons with photoshop or iconworkshop, resize them and upload to carbide. That's all, me too do the same.