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@ericgregorich Just like @manton, I've used custom style blocks in some of my plug-ins. It's fast and easy, but if you have a lot of CSS, you might want to break it out to separate files. Hugo has a robust assets pipeline, so you can: have dynamic (template-based) CSS files, bundle multiple CSS files into one, process SASS and SCSS, run minification, and more.
There's a minimal example on the page Creating a resource from a template.