Ok but your description of events doesn't really tie up with the result.
Your custom theme, seeing as it worked for some time, can be pushed to the back burner in the culprit stakes for now.
A Banner brick, typically, will result in an error screen after the health warning screen. Messing with wads and channels is the prime candidate for banner bricks. Channels will install perfectly even when they are bad, that's the trouble.
Before embarking with messing around with wads did you have precautions such as preloader and bootmii installed to protect yourself in the eventuality of a problem?
Hold reset as you power your Wii on. Does anything happen?
Check for blue light flashing twice in 1 second (approx) when you power on. Does it do this?
Savemii can help banner bricks. Hold out until it arrives and see if you get the recovery screen when you power up. This will be mostly black but have a number such as 4.1(Eur) in the bottom right corner.