The docfx documentation site shows how to customize the various .partial files used in the default template (see https://dotnet.github.io/docfx/tutorial/walkthrough/advanced_walkthrough.html).
I've followed those instructions, creating a new template folder and putting a modified styles\main.css file in it, and also took the step under the Apply the Template section where the custom template name is specified in addition to the default:
"template": [
"default",
"templates/<name of your your HTML template folder>"
],
However, when I build the site, the _site/styles/main.css file doesn't have my changes.