Remove `background-color` from tab

When using a theme in dark mode, the background color of the tab makes it illegible.


This is due to the CSS:

```css
.http-example .caption.selected {
background-color: #fff;
}
```

To work around this issue, I added a custom style to Plone Sphinx Theme in https://github.com/plone/plone-sphinx-theme/pull/29.

```css
.http-example .caption.selected {
background-color: unset;
}
```


However, I can see why you would want to set the background color for themes that have a light grey content background, such as Read The Docs Sphinx Theme.


Here's how it would look, and the difference is negligible.


If you think this would be a good improvement, I can submit a PR.

See also https://github.com/plone/plone.restapi/pull/1815