Equipping a cosmetic theme recolored text, fb-* utility surfaces, and
body, but the left sidebar's navy radial wash stayed on its default — so
the interface read as "only the fonts change, not the backgrounds".
Cause: #v3-sidebar is painted with a hardcoded radial-gradient in v3.css
and carries no fb-* utility class, so theme-core's per-utility override
loop never reaches it (#1e293b == default card, #0f172a == default bg).
Extend cssFor() — which already special-cases body — to re-point the
sidebar gradient at the theme, gated by html[data-fb-theme] so the
default (no-theme) look is untouched. Only background-image is overridden,
preserving v3.css's background-attachment:fixed.
Verified in Chromium against the real tailwind.min.css + v3.css +
theme-core.js: default = navy gradient (unthemed), apply() recolors the
sidebar to the theme's card->bg stops, apply(null) reverts to navy.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
