Fuck this! I just got scared shitless for no reason because I accidentally tried installing a theme (didn't mean to try it at that moment, I thought it would just enter the damn folder!) and the theme never seemed to finish installing (I waited for like 20 minutes or so), so I had to force shut down the console and I thought for sure the theme had bricked my Wii U....
I searched for all kind of solutions (possibly found 1), but I was going crazy not knowing whether or not this was actually fixable and I needed to know If I could atleast boot into the environment Loader to have a chance at fixing things, so I tried turning it on again....And the thing works no issues. Theme wasn't applied but I couldn't care less!
Fuck, my heart....I have a completely fresh backup but only the kind that can be applied with a hardmod, which I DON'T have the skills to do! Time to look into installing isfshax for brick protection....
If anyone actually bricks their Wii U with this, the following solution might work:
https://gbatemp.net/threads/fix-bad-wii-u-theme.638054/