It's possible to get the PDF to look fairly close to the default HTML5 theme, but there are some limitations. At the very least, you can start by aligning the fonts and colors, then go deeper.
For a while, I had a theme in Asciidoctor PDF that (somewhat) matched the default Asciidoctor stylesheet, but I pulled it out because the theme system is still changing too much for it to make economical sense to maintain both themes. Once we move closer to a final 1.0.0 release, I'll consider adding it back in.
I have experimented with some other themes and I plan to publish them to GitHub as soon as I get a chance. The exercise of creating themes helps me to better understand where the theme system needs to be enhanced.
So, long story short, anyone willing to experiment with creating themes for Asciidoctor PDF is part of the solution :)
Cheers,