Highlight message targeted by anchor

Could we perhaps add something like this to the styles so that it is easier to see which message a permalink is referring to?

```
div:target {
background-color: #9fb;
}
```

https://github.com/indieweb/chat.indieweb.org/blob/main/public/assets/styles.css

Start in public/assets/styles.css, the file linked by the issue, and inspect the existing message styles. Add the proposed target highlighting so a permalink's message is visually distinct, then verify that a message anchor highlights the targeted div without affecting other messages.

Written by the indexing model from the issue text.