Styling `:focus-within` on `<details>`

Currently tabbing to a `` does not show the usual `:focus-within` outline (technically, tabbing to the ``)

This could be fixed with

```
details:has(summary:focus-within) {
border-color: var(--accent);
outline: .2em solid var(--accent);
}
```

Use the issue's details:has(summary:focus-within) selector as the entry point and locate the existing and styles. Test keyboard focus on a in a browser, then confirm that the details element shows the accent border and outline while its summary is focused.

Written by the indexing model from the issue text.