Code folds do not suggest interactivity

Following up on a [discussion on Slack](https://lit-and-friends.slack.com/archives/C03PF4L4L/p1619158766416200), it turns out the `...` as a code fold mark does not suggest interacting by clicking on it.

**Proposed solution**
A light border might already improve this, as per this comparison:

Without border (current) | With border
------------ | ----------
 | 

I'm sure this can be even further improved -- I'm not so good at styling things. Still, here's the entire change I applied:

```css
.CodeMirror-foldmarker {
font-family: sans-serif;
/* I added these below */
border: 1px solid darkgray;
border-radius: 2px;
padding: 0 4px;
}
```