Example does not work in dark mode

### Describe the bug

This example uses white-on-white text which is difficult to read.


### Reproduction

Enable dark mode on computer.

Access https://vue3.examples.histoire.dev/story/src-components-eventbutton-story-vue?variantId=_default

### System Info

```shell
N/A. Using Chrome web browser.
```

### Used Package Manager

npm

### Validations

- [x] Follow our [Code of Conduct](https://github.com/histoire-dev/histoire/blob/main/CODE_OF_CONDUCT.md)
- [x] Read the [Contributing Guidelines](https://github.com/histoire-dev/histoire/blob/main/CONTRIBUTING.md).
- [x] Read the [docs](https://histoire.dev/guide/).
- [x] Check that there isn't [already an issue](https://github.com/histoire-dev/histoire/issues) that reports the same bug to avoid creating a duplicate.
- [x] Check that this is a concrete bug. For Q&A open a [GitHub Discussion](https://github.com/histoire-dev/histoire/discussions).
- [x] The provided reproduction is a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example) of the bug.

Start by opening the provided EventButton story reproduction in Chrome with dark mode enabled and locate the source for story/src-components-eventbutton-story-vue. Compare the example's text and background colors in dark mode. Done means the example's text is readable without white-on-white contrast.

Written by the indexing model from the issue text.