Starting PDF pages with an offset

I'd like to make an adoc for only part of a doc (no title page or TOC) to be merged with an existing PDF file.  So I'd like to be able to specify a starting page (offset) in the theme file, so I can later merge PDF files and the pages will be continuous.

I'm not aware of such a feature.

But another feature might help you to achieve something similar: You can import PDF pages into asciidoctor-pdf at generation time:
https://github.com/asciidoctor/asciidoctor-pdf#importing-pdf-pages

One way I just found...

Add a 'declaration' of a counter like so, such that it doesn't show in the output.  Make it one less than the start page.  'define-steps' is not used:

:define-steps: {counter:steps:6}

Then in the style YAML, for example in the footer content use:

{counter:steps}

which will start at 7 in this case.