/* In here place all the CSS for the image gallery */
Place all the files in a directory and render it like so:
$ asciidoctor MyTestImageGallery.adoc
Then begin debugging the code generated. You would have to examine the output of the MyTestImageGallery.html and adjust the class names in CSS in the docinfo.html file to match. And there may not be enough tags and classes to grab onto to make this work. So it might require a lot of tweaking the CSS image gallery code. It might be better to shop around for other CSS image galleries that have simpler code that might match the Asciidoctor HTML output better.