CSS: hsl() & rgb() doesn't minify when inside of custom properties

I'm not sure if it's intended, but this:

```css
html {
    --color: hsl(0 17% 50%);
    --foo: rgb(1 2 3);
    background-color: hsl(0 2% 100%);
}
```

minifies to:

```css
html{--color:hsl(0 17% 50%);--foo:rgb(1 2 3);background-color:#fff}
```

instead of:

```css
html{--color:#956a6a;--foo:#010203;background-color:#fff}
```