Cannot use simple vars value for setting CSS variable

I tried to set simple vars as CSS variable, but it doesn't work:

```css
$blue:   #056ef0;

:root{
  --color: $blue;
}
```

It will gives an error like this:

```
Syntax Error: SyntaxError                                                      friendly-errors 20:24:33

(1:1) <css input> Unknown word

> 1 | $blue
    | ^
```

Is this by design?