Use a neutral background color for callout

[ggrossetie]

Is there a particular reason to use a green background on callout?

asciidoctor-reveal.js/templates/asciidoctor_revealjs.css.slim

Line 53 in 48b76bc

.conum[data-value] {display:inline-block;color:#fff!important;background-color:rgba(50,150,50,.8);-webkit-border-radius:100px;border-radius:100px;text-align:center;font-size:.75em;width:1.67em;height:1.67em;line-height:1.67em;font-family:"Open Sans","DejaVu Sans",sans-serif;font-style:normal;font-weight:bold}

The default stylesheet is using a black background with a with font:

.conum[data-value] {
  color: #fff!important;
  background: rgba(0,0,0,.8);
}

https://github.com/asciidoctor/asciidoctor/blob/master/data/stylesheets/asciidoctor-default.css

I think we should use a neutral color because depending on the chosen reveal.js theme, this green background might look out of place.

[obilodeau]

No particular reason. I'm pretty sure I just inlined Asciidoctor's (at the time) CSS.

Syncing that style with upstream is reasonable to me.