Scaffolds a site-local theme from an official one.
Usage
mix cherry.gen.theme NAME [--from THEME] [--source DIR] [--json]Copies an official theme (--from NAME, default: default; the
error for an unknown name lists them all) into themes/NAME/ —
manifest, templates, stylesheet, islands; templates a CSS-only theme
inherits are materialised into the copy — and
renames it. Point cherry.exs at it with theme: "themes/NAME" and
every file is yours; the swap contract keeps the site building
throughout.