It's not.. `light-color` refers to the colour you want presented if the user's preference is for a "light" theme.
It's not declaring the relative brightness of the two colours - I agree it could be documented a little less ambiguously with something like `light-theme-color`.
It sets the foreground/text color because it's used with the `color` property (https://developer.mozilla.org/en-US/docs/Web/CSS/color). You can use it to set the background color too in which case you'd do `background-color: light-dark(white, black);`
