What's the official/proper way to create theme for Vivo phone? (.itz package file)

So my question is what's the proper way to made theme for Vivo phone in general? My special case is Vivo 11i.

I search around internet and learnt some tips& tricks involved replace xml, extraction, compression to download and edit Vivo theme from official Vivo theme market (online). They work to some extents though, still there's limitations.

But a thing still stays in my mind, that's how do people made and publish Vivo theme at the 1st place?? I searched everywhere and there's no official...

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