There are a a few ways to fix this sort of issue and they all require a mod chip. The easiest way is with an auto-boot disc of an application to uninstall the theme. However, this only works if you have the trucha bug in the system menu IOS (unless you patched IOS 70, you do not have the trucha bug in it and can't use this method).
The other option is to run a game with an exploit in and use that exploit to run the uninstaller for the theme. The issue here is getting the game to run. If you already have Indiana Pwns on your wii, you can burn an autoboot Lego Indiana Jones game, use the exploit, and run the uninstaller.
If you do not have the trucha bug in IOS 70 and you don't have the Indiana Pwns save on your wii, your only option is to use a mod chip that can autoboot and region convert games and run an NTSC-U copy of Brawl. After getting Brawl to run, you can use Smash Stack to run the uninstaller for the theme. The issue with this is that you would need to configure the chip to autoboot the game, and this probably requires the wii to be functional. You may be able to configure it in another wii, but, if the chip saves its settings in volatile memory, they will be deleted before moving the chip into the bricked wii. If you can get Brawl to start, you can fix your wii. However, as far as I know, no one has successfully done this.