[Feature Request] Support CSS Variables in color variables

### Existing Component
Yes

### Component Name
theme-chalk

### Description
It would be awesome, if css variables can be used for setting the theme colors.
This would enable the immediate change of theme colors on the fly even after compillation.
unfortunately the `mix($color1, $color2, $weight)` method does not support, that, but it should be possible to use `rgba(var(--primary-color), $weight)` method.