Nit, might be a big cleaner like:

```js
  proto._setSkin = function(skin) {
    this.header.setAttribute('skin', skin);
  };
```

Stylesheet will need some changes to use `[skin="foo"]` selectors, but this is more in-line with how our other custom-elements are styled.

When we have `:host` selector, we can remove this function completely, as all styling will be able to use the attribute set on the main element.