[Documentation] Utilize color-mix() to allow arbitrary color CSS variables with opacity

[mordechaim]

According to the documentation you must only set the color channels for color CSS variables in order for <alpha-value> to properly work.

Well, with color-mix() now supported by all major browsers there's a nice way to add opacity to any color:

colors: {
    mycolor: 'color-mix(in srgb, var(--variable-color) calc(<alpha-value> * 100%), transparent)'
}

This is based on this blog. I've tested it and it works properly.

I think it'll be helpful to include it in the documentation.