I wanted to tweak moderncv. My initial reaction was to run rendercv create-theme moderncv_custom but this generated - I beleive - a classic theme.

Converting the classic theme to moderncv is more work than just initializing from moderncv.

Maybe with this syntax:
rendercv create-theme --source=moderncv moderncv_custom

Then I found out from https://docs.rendercv.com/user_guide/how_to/override_default_templates/#two-methods that I could just override bits of the existing templates via creating a folder with <THEME>/Header.j2.typ, ie in my case moderncv/Header.j2.typ.

Feel free to close.