IColor.toCSS() emits var(--sxl-sys-color-windowtext, …) for system/auto colors, so the browser
resolves them in place — per DOM subtree, exactly like CSS. These defaults make the full
--sxl-sys-color-* palette a visible, overridable contract: a host (MUI app, VSCode extension)
themes the grid (or an individual widget) by overriding the variables on a container. Using a
:root stylesheet (not inline styles) keeps specificity low so host overrides win.
Injects the default
--sxl-sys-color-*CSS variables — the built-in (baked) system color definitions — as a low-specificity:rootstylesheet (idempotent).