Fix CSS scoping for CSS variables

```
:root {
  --color: red;
}

label {
  color: var(--color);
}
```

src/Previewer/Internal.js

@@ -257,7 +257,7 @@ function scopeStylesheet(style, id) {
   for (const node of ast.nodes) {
     if (node.selector === "window") {
       node.selector = `#${id}`;
-    } else if (node.selector) {
+    } else if (!node.selector.startsWith(":")) {
       node.selector = `#${id} ${node.selector}`;
     }
   }