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Unless the design implemented looks nothing like either of them and they don't give you a starting point, of course. Then it's better to simply not include them

For sure, you do not need to have those themes installed. You can use anything you like. starter and Watea are meant to help you get started, not to provide a full fixed theme that everyone should use 🙂

We have 4 sites all using similar themes, so i generally just grab the bits i need and create seperate git repos for each them that install via composer. For the latest one i created it from complete scratch (not sure what starter and Watea are!) but im getting ready to commit everything. Would be much easier to keep it in one repo, but if its still preferred to go via composer I will do that.

I would strongly suggest to have all the common components in a separate git repository, which you can include in your composer.json

And then use a inheriting theme that overrides the necessary bits in the core application repo

Watea and starter are CWP built themes, you can find them on GitHub... 🙂

It provides a base theme to work off, and you can inherit from that 🙂

But if you don't want that, you are obviously free to use whatever you like