Support for CSS variables

Hey, thanks for a great library!

Do you have any plans on supporting CSS variables? Passing those to the theme doesn't work because the canvas element struggles with recognizing those (by design).

The workaround is to usually just import those colors from some theme provider or do something like `getComputedStyle(document.documentElement).getPropertyValue('--color-blue-100');`, but I see a potential opportunity to simplify things for consumers.

Was this ever considered? Any strong opinions as to why baking this feature in might not be a good idea?

Thanks!