```js
// YourComponent.stories.js

import YourComponent from './YourComponent.svelte';
import MarginDecorator from './MarginDecorator.svelte';

export default {
  component: YourComponent,
  decorators:  [() => MarginDecorator]
};

// Your templates and stories here. 
// Don't forget to use the component you're testing and not the MarginDecorator component

```

```html
<!-- MarginDecorator.svelte -->

<div>
  <slot/>
</div>

<style>
  div { 
    margin: 3em;
  }
</style>
```