`useTheme`, the accent override, the retint helper and the OKLCH math reached
the SDK without reaching this page, which still documented `THEMES_AREA` alone.
Registering a theme only lists it in the picker, so the natural reading was that
plugins cannot switch themes at all — and the one person who tried concluded
exactly that and patched the app instead.
Documents the selection half: the hook for components, `requestTheme` for
callbacks with no component around them, and a Theming row in the export table.
The agent-facing reference gets the same note, since it never covered themes.
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