I attempted to reinstall the software and it worked briefly, but promptly broke again.
The dev console shows a large number of Warnings, all along the lines of
DevTools failed to load source map: Could not parse content for file:///C:/Program%20Files/Joplin/resources/app.asar/ResourceScreen.js.map: Unexpected end of JSON input
However no errors, and no mention of the userstyle.css