Lets a trusted local extension contribute a custom skin to the NATIVE
Settings -> Appearance picker (instead of a parallel theme switcher).
window.registerHermesSkin(descriptor) validates + sanitizes the skin in
core, once, so every theme extension inherits the guard:
- token names restricted to a documented allowlist
- values sanitized against a strict color/dimension regex (rejects url(),
expression(), semicolons, braces, and other CSS-injection vectors)
- reserved built-in skin keys cannot be overwritten
- descriptor with no valid tokens is refused
- idempotent re-register (updates in place) for a live theme editor
Injects a managed <style> for the :root[data-skin] tokens, appends to
_SKINS + _VALID_SKINS, refreshes the picker, and re-applies a persisted
selection that was waiting on a late-registered skin.
Unblocks theme-pack and theme-creator extensions. Documented in
docs/EXTENSIONS.md. Regression test drives the real registration +
sanitization logic via a Node harness (adversarial input).
Full suite: 10874 passed, 0 failed.
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