You need to set the attributes pdf-stylesdir and pdf-style for it to locate the theme file.
> I want to achieve something that has the same level finish as the Docbook output before proposing it to my colleagues.
It's not going to be possible at this point to polish it to the level you can with DocBook. Asciidoctor PDF is intentionally more simple than the DocBook Apache FO PDF output at the moment. You could say that it is more opinionated...though the truth is it's just not as mature.
To really tweak the output of Asciidoctor PDF, you have to hack / override the Ruby code and that's just not something that's easily linked up with a Maven build yet. We'll find our way there, but it's going to take time.
Cheers,
-Dan