Also, if you are on a newer windows PC, make sure you view what the file extention actually is, I had a problem with this last year where I named something e.g. theme.mtz.mtz because windows was not showing me the extention, also, I had success a few days ago changing a theme from .mtz to .zip, extracting it, editing it, zipping it back up, and changed it back to .mtz and it worked just fine
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