Custom CSS for OBS input field and preview

An input field for CSS on the homepage. Real-time application to the preview. Easy copy to the "Custom CSS" field in OBS.

Don't forget to pre-fill it with the same default OBS uses[^1]:

```css
body { 
  background-color: rgba(0, 0, 0, 0);
  margin: 0px auto; overflow: hidden;
}
```

[^1]: https://obsproject.com/kb/browser-source