Okay. I will research this a bit more.
I do not want to add it to one markdown file, I want it added automatically to all markdown files during processing. The ideal way to do this would be to have a preference where I could have a file with content that is directly included in the HEAD of the Markdown rendered as HTML. So that rather than linking to a local file I could add content directly, including linking to local files, remote files, etc.
That Style Sheet preference produces this in the HTML.
<link type="text/css" rel="stylesheet" href="file:///Users/mmynsted/style/lit-plus.css"/>
This is a local file reference that is often blocked by web browsers.
Better would be something like this.
<link type="text/css" rel="stylesheet" href="http://localhost/css/lit-plus.css"/>
This is not blocked.
Best would be to directly include content to all Markdown files durning processing.
Like the proposed solution here → Please make it possible to include JS/CSS files in markdown rendering from Application Support folder
I wish there was a hidden preference for this. As is I will just have to deal with the issue of most browsers not supporting loading local resources. It can be overridden in modern Safari, but not Orion, etc.