Parse a shadcn/tweakcn theme CSS string into light + dark ColorTokens. Dark is read from a .dark /
html[data-theme="dark"] block when present, otherwise deterministically derived from light. Returns null when
the CSS has no recognizable :root shadcn token block.
Parse a shadcn/tweakcn theme CSS string into light + dark ColorTokens. Dark is read from a
.dark/html[data-theme="dark"]block when present, otherwise deterministically derived from light. Returns null when the CSS has no recognizable:rootshadcn token block.