Flexible color linking#137

Ability to reference Light/Dark color variants from any other color variable (even if the color variable has only light variant).

For example there is a brand core tokens. Because it is a core tokens there is no such thing as ligh/dark varinat for them. Next we have a theme tokens which reference to values in the core palette (theme tokens have light and dark variants).

Core tokens:

–red-100: red;
–blue-100: blue;
–green-100:green;

Theme tokens:

–bg-button: var(–red-100) var(–blue-100)

Right now there is no way to reference to the blue-100 value, as a dark theme variant. The only solution is to add dark variant to the –blue-100 core token and duplicate same value for light and dark. Also it is not possible to reference color if it was created in the variable group.