Minify colours in CSS variables

[TomStrepsil]

Although this (from docs) works:

csso.minify('.test { color: #ff0000; }').css
// .test{color:red}

...the same doesn't appear to apply to css custom properties:

css.minify(':root { --test: #FF0000; }').css
// expected: :root{--test:red}
// received: :root{--test:#FF0000}