Relative Color Syntax doesn't work with `light-dark()` CSS function

[Roman Czerkies]

Created attachment 471355 [details]
Bug reproduction with and without `light-dark()` CSS function

When Custom Properties is assigned from the `light-dark()` CSS function, the CSS Relative Color Syntax doesn't work.

``` css
:root {
    --background-color-light-dark: light-dark(oklch(65.74% 0.2 34.41), oklch(35.74% 0.2 34.41));
}

p {
    background-color: oklch(from var(--background-color-light-dark) l c h / .5); /* doesn't work */
}
```

Bug reproduced with and without `light-dark()` CSS function in the attachment.

[Radar WebKit Bug Importer]

<rdar://problem/128260951>

[Roman Czerkies]

Fixed in this issue: https://bugs.webkit.org/show_bug.cgi?id=245970#c6