Scaffolds a site-local theme from an official one.
Usage
mix cherry.gen.theme NAME [--from THEME] [--source DIR] [--json]Copies the official theme (--from default, or cherrybomb) into
themes/NAME/ — manifest, templates, stylesheet, islands — 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.