Override navigationCard styles

Hi!
Thanks for this awesome library!

I have image on background in my app.
````
const style = {
image: {
flex: 1,
width: null,
height: null,
resizeMode: 'cover',
},
}

const Root = (props) => (

{props.children}

);

````

So, if I'm try to using ````, then i have gray background instead image. I found in ``styles.js``
````
navigationCard: {
...absoluteFillObject,
overflow: 'hidden',
},
````

Making this
````
navigationCard: {
...absoluteFillObject,
backgroundColor: 'transparent',
overflow: 'hidden',
},
````
solve my issue.

Is there a way to override those styles?

### Version

| Dependency | Version |
| ------------------- | ------------------- |
| react-router-native | 2.0.0-beta.0 |
| react-router | N/A |
| react-native | 0.37.0 |