Couple of things I noticed
1. When viewing the ISIS website on a mobile device (or browser with narrow width) the "nav" items collapse to be replaced by the 3 horizontal lines icon. This is presumably intended to then show a dropdown of the "nav" items. But it doesn't do anything seemingly.
My experience of asciidoc templates is very limited so can't tell you what is wrong (though was investigating writing a DN variant and hit the same problem, so would be interested in the solution!), and you seem to have used Deltaspike as a starting point for yours (theirs also has the same problem). A site that does give the three horizontal lines, and subsequent dropdown is http://vertx.io/docs/vertx-core/java/ in case its of use.
2. Table headings are shown with white text on a light grey background, hence virtually unreadable. For example
Setting th.tableblock background-color and color should fix that.