Extracted from JENKINS-30836:

The path for custom CSS/JS files is confusing as the absolute file path is required (/home/jenkins/jobs/build/workspace/trunk/file.js) there. In contrast, the JGiven result json files path is relative (/trunk/jgiven-reports)

In all other plugins I know, the paths are always relative, so the best solution is to also use relative paths for custom CSS/JS. Maybe it would also nice to support http links.