I know that I can use :sectnums: all after the title to enable section and subsection numbering. But is it possible to define this in the theme yml? I have a lot documents which use the same theme file, and it would be nice to have it defined only once.
The PDF theme.yml does not support setting AsciiDoc attributes, so that approach won't work.
However, you can specify attributes on the command-line:
asciidoctor-pdf -a sectnums=all file.adoc
It might be better to create an AsciiDoc file that only defines attributes, say attributes.adoc. You would specify any attributes that you want to use globally:
:sectnums: all
:toc: left
Then, in every document that needs to use those attributes:
= My document
include::attributes.adoc[]
Lorem ipsum...
Once you've done that for all documents, adjusting the attributes requires editing only the attributes.adoc file.