Theme and plugin

Slowly is a plugin. The builder, the blocks, the Themer, and the global design system all come from it, and it runs on any block theme.

It also ships a theme called Slowly, a clean block theme to start from. You can run that, the theme you already have (the old Cwicly theme included), or any other block theme like Twenty Twenty-Five. The plugin behaves the same on all of them.

Templates and menus, though, belong to the active theme. WordPress stores them against whichever theme is on when you build them, so switching themes leaves them behind with the old one, and moving them across is a manual job.