To be honest I've never installed any theme for the Wii System Menu. It's not 100% safe as far as I know, even if Priiloader and Bootmii can offer a good degree of brick protection, but I prefer to avoid messing up with my system files on real NAND whenever I can (apart from the needed cIOSes and channels). I've heard of bad bricks in the past for some people.
In my case, I use Wiiflow + Rhapsodii Shima as an actual replacement for the system menu, configuring Priiloader to boot straight to Wiiflow. With RS I can access to everything that I could run from the System Menu, without actually install anything on Real Nand. And with the wonderful Wiiflow interface.