# 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](032-greyscale-theme-rework.md), 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](032-greyscale-theme-rework.md)
- [008 — Settings model](008-settings-model.md) (`CssEntry` import/link flow)