Evaluating a css-variable by a declaration-block

Accidentally, I saw such CSS:

:root {
  --flex-theme: {
    border: 1px solid #00F;
    padding: 3px;
  };
}

That was the first time I saw a complete declaration block ({border: ...; padding: ...;}) assigned to a CSS variable (--flex-theme).

How would this be usable?