ottyl/tickets/033-custom-theme-download-upload.md

033 — Downloadable and re-uploadable custom themes

Summary

Let a user export the active theme's --c-* variable values as a file, edit it, and load it back in as a custom theme — reusing the CSS import/link machinery Settings already has (CssEntry, import_css/link_css in src/settings.rs) rather than inventing a parallel storage mechanism.

Scope

• Download: given a theme name (one of the built-ins from 032, or a previously-uploaded custom one), produce a small CSS (or JSON) file containing just its --c-* variable values, served for download from the webapp's theme picker.
• Upload: accept such a file back, validate it defines the expected set of --c-* variables, and register it as a selectable custom theme — persisted via the existing import_css-style flow (a new CSS entry scoped under a html.<custom-name> selector, or a dedicated theme entry if that shape doesn't fit CssEntry cleanly — decide during implementation which reuses more of the existing settings model).
• Linked custom themes: since CssEntry::Linked already models "reference a URL, don't own the content," a custom theme can also be registered by URL instead of upload, consistent with how linked CSS works today.
• Theme picker UI: list built-in themes alongside any uploaded/linked custom ones, each with a download action.

Acceptance criteria

• Downloading a theme produces a file that, re-uploaded unmodified, recreates the same theme.
• Editing a downloaded theme's variable values and re-uploading it registers a distinct selectable theme without touching the built-ins.
• A linked (URL) custom theme behaves like linked CSS today: not deletable via the "remove" action that applies to uploaded ones, per CssError::NotImported's existing semantics.

Depends on

• 032 — Greyscale theme rework
• 008 — Settings model (CssEntry import/link flow)