```js
// Button.stories.js

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

export default {
  /* 👇 The title prop is optional.
  * See https://storybook.js.org/docs/7.0/svelte/configure/overview#configure-story-loading
  * to learn how to generate automatic titles
  */
  title: 'Button',
  component: Button,
  decorators: [() => MarginDecorator],
};

// Your 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>
```