bb-theme-authoring

# Authoring a bb theme A bb theme is one CSS file that overrides the app's CSS custom properties. bb discovers custom themes on disk and the **Theme Preview** panel shows every resolved token, its validation state, and an app mock that repaints as you edit. Work in a split: your agent thread on one side, Theme Preview on the other. Nothing needs a restart. Theme Preview is visualization and validation only. It never creates, writes, forks, repairs, or deletes a theme resource. Create and update `theme.css` from the separate agent thread; use the existing `bb theme` commands to locate, inspect, and activate themes. Validation reports inconsistent or inaccessible results, but it never changes authored values. ## Where themes live ```sh bb theme dir # the custom-theme directory, e.g. ~/.bb/theme ``` One directory per theme, one file inside it: ``` <theme dir>/<name>/theme.css ``` `<name>` is the theme id: lowercase, letters, digits and dashes, a single path segment. Create the directory and the file and it is listed immediately — the Theme Preview dropdown picks it up on its next catalog refresh while the panel is open, and `bb theme list` shows it at once. ## File shape Two top-level blocks. Light values in `:root, .light`, dark values in `.dark`. Every declaration is a `--token: value;` custom property. ```css :root, .light { --canvas: #f4f4f4; --ink: #0a0a0a; --primary: #2e6f95; --primary-foreground: #ffffff; } .dark { --canvas: #1a1a1a; --ink:...