Optionaloptions: CSSSystemLookupOptionsA host (for example the VSCode extension or a MUI app) defines variables like
--sxl-sys-color-windowtext / --sxl-sys-color-window (optionally --sxl-sys-color-windowtext-dark) and
installs this as the concrete default lookup (see installSystemColorLookup). Note that
grid rendering does not use this — it emits CSS var() directly (see useSystemColors);
this lookup is for toRGBA-based consumers (color pickers, contrast).
Resolution uses a hidden probe element: the variable is applied to the probe's color
and the computed value is read back. This forces the browser to fully resolve var()
chains (e.g. var(--vscode-editor-foreground)) and CSS system keywords (Canvas,
CanvasText) down to an rgb(...) value — neither of which is reliable when reading the
custom property string directly.
Results are cached per (name, darkMode). Returning null (variable unset or no DOM) lets
the color fall through to its lastClr / built-in baked value, so this is always safe to
install — including in headless / Node environments.
Creates a IColor.SystemLookup that resolves system colors (e.g.
windowText,window) from CSS custom properties on the document.