Remove `background-color` from tab

[stevepiercy]

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

This is due to the CSS:

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

To work around this issue, I added a custom style to Plone Sphinx Theme in plone/plone-sphinx-theme#29.

.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 plone/plone.restapi#1815