A better idea is just to put a small hole in your case somewhere and run the mains cord out through it. You can even easilly enough open up an extra PCI blank and run the cord through there.
It's not going to be as simple as just stripping the wires and jabbing them into the appropriate holes on a molex connector. There's lots of different stuff going on with electricity - Ampage and watts to be specific - than just voltage.
As a rule, don't try to power things off your PSU that aren't made to be powered by it. There's hundreds of lighting solutions available made for the purpose, that come with inverters that allow them to run direct off a molex, so if being powered internally is a real worry to you (Why?) then just buy a proper...