Multiple defaults for CSS Custom Properties

Consider this case

```js
class MyElement extends HTMLElement {
constructor() {
super().attachShadow({ mode: 'open' }).innerHTML = `

:host { color: var(--my-color, red); }
@media (prefers-color-scheme: dark) {
:host { color: var(--my-color, blue); }
}

`;
}
}
```

Should I get
```json
"cssProperties": [
{
"name": "--my-color",
"default": "red"
}
]
```

Or

```json
"cssProperties": [
{
"name": "--my-color",
"default": "blue"
}
]
```

And if I get either, that's not really correct is it?

We should add a schema facility for declaring multiple defaults and their conditions