Try to switch PD4ML into debug mode with pd4ml.enableDebugInfo() and inspect STDOUT/server’s log which URL it actually tries to load.

If in your HTML document references are defined as /path/to/stylesheet.css and in the debug output they appear prefixed with file:, pd4ml.useServletContext() call should help.

But it is a better idea to reference CSS stylesheets with relative paths (like ../mycss/stylesheet.css).