Suggestion: move from `hsla()` and `-hsl` variables for transparency to `color-mix()` 

The `color-mix()` CSS function, now generally available, provides semantics for mixing colors without (a) mandating that the colours be expressed in HSL and (b) without forcing the duplication of variables (e.g. `--foo` and `--foo-hsl`). 

We could use this to streamline blueprint-styler (which, by the way, I love!)

Would you all be open to a PR? 