CSS custom properties don't have to be valid CSS properties
A custom CSS property doesn't have to be a valid CSS property on its own.
For example, you can define a color as three numbers between 0 and 255 for red
green and blue. A list of three numbers is not a valid color definition in CSS.
But you can use them in the
rgb() function to get a valid color. Using the
rgba() function it's possible to get variants with transparency of our
--color-primary: 94, 142, 194;
border: rgb(var(--color-primary)) 2px solid;
background-color: rgba(var(--color-primary), 0.4);
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