Hi,
The question is not about code higlighting but rendering, so when you look at an
.adoc file it looks like what it would look once transformed into html.
Just like, when you are looking at a readme.md, you don't see the markdown
syntax (unless you specifically ask for it).
For this you can create a local test (I think how to do this is explained in the docs' pagure) or use an editor that create a preview like atom with the asciidoc plugin or asciidocfx. More options can be found on the asciidoc site [1].
Hope this is helpful,
Eduard Lucena