Cannot pass styles to alpha component
Desired API:             

```
const styles = reactCSS({
     'default': {
        gradient: {
          background: '#000',
        }
    });

<Alpha
  styles={this.style}
/>

```

Currently you cannot style the alpha component at all aside from the pointer. My particular use case is trying to turn the gradient into a solid color bar.

Any suggested ways to implement this?
