C'mon, man. Just buy a theme, they are cheap as hell and it's worth it to have constant updates. Get a membership with elegantthemes, a whole library for a small monthly fee with all kinds of great shit.
If you pirate a theme and use it, yeah, you could get fucked. Someone could easily hide links to their own site, automatically use their adsense ID instead of yours, and in general make your site insecure for their own gain. And you will have deserved all of it for being such a cheapass. If you don't know shit about themes, it's even more of a reason not to pirate them.