As long as you follow the instructions, you'll be fine. I installed my vWii theme forever ago and it couldn't have been easier. Plus, if you brick your vWii, you can easily recover it from the Wii U side of the system (at least that's my understanding, I've never had to recover the vWii from a brick, but its supposed to be simple and easy). I will say that if you've installed Priiloader to your vWii already that you should not install a new menu theme at that point, you can install a theme before installing priiloader, but installing one after you've installed Priiloader, even i you do everything right, it will also lead to the vWii bricking.