Start Stackable on activate is Default (designed blog, no import). Site Kits are plugin packages that write pages, a theme styles overlay and/or shipped variation, header flags, and optional GDS.
The catalog may show Default as a label ("you are on the theme"). Clicking it does not import and does not reset an imported kit. Never import on theme or plugin activation.
theme.styles in the contract lets a kit restyle the header and footer without shipping a new file in the theme zip.
Slugs must match the theme token contract.