just put your theme-packs in the packs folder
for infos like author, tags, name you can add pack.meta to your packs
example:
name=Some themes
author=Some author
tags=Tag1,Tag2,Tag3
in rboard manager v3 you can add the raw list.json as repo (https://raw.githubusercontent.com/GboardThemes/PackRepoBeta/main/list.json)