it should be easier to make Wii U themes without downloading an already existing .zip folder and installing a patch, then hoping that your Wii gets doesn't get softbricked, it would be easier if you were able to edit themes like the Usagi / Kame theme editors for the 3DS!
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First of all, "making" themes doesn't mean you are installing them. If there is already a zip then delete it. What has the Wii to do here, Maybe you mean "WiiU". and no, You CAN'T "softbrick" using the SDCafiine method to install wiiu menu themes.
And finally, Theres a tool called "Switch Toolbox" that isn't specifically for editing themes but you can use it for editing WiiU Menu files, And don't ask me how, ur just noob.