Detect unused css custom properties (aka variables)

<div>sdlfkj</div>

::root {
  --reddish: red;
  --unused: cornflowerblue;
}
div { color: var(--reddish); }
span { color: blue; }

::root {
  --reddish: red;
  --unused: cornflowerblue;
}
div { color: var(--reddish); }