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How do I include my css in the asciidoctor workflow that would override/tweak a css value in the themed/factory styles?
You would create a stylesheet that uses @import to include the original stylesheet, then add your customizations below it. However, Asciidoctor will not embed the original stylesheet into the document in this case because it doesn't follow @import directives.
What I recommend is simply making a script that collates the two stylesheets together, then pass the result to Asciidoctor if you want to have the entire stylesheet embedded.
-Dan