Optionally installs a global concrete CSS system-color lookup, for toRGBA-based
consumers (color pickers, contrast via toBlackOrWhite) that need an actual number rather
than a CSS var().
Rendering does not need this — it uses CSS var() (the browser resolves --sxl-sys-color-*).
This is opt-in for hosts that want concrete system colors resolved from the host theme. Returns
the lookup so the host can invalidate() it when the theme changes.
Optionally installs a global concrete CSS system-color lookup, for
toRGBA-based consumers (color pickers, contrast viatoBlackOrWhite) that need an actual number rather than a CSSvar().