Default theme no longer working after upgrading to v2

I fixed it with the following:

```ts
<Highlight code={code} language={codeLanguage} theme={{ plain: {}, styles: [] }}>
```

This was causing errors:

```ts
<Highlight code={code} language={codeLanguage} theme={undefined}>
```

The error was:

> Cannot destructure property 'plain' of 'theme17' as it is undefined.