1. Try to reformat the single-line CSS stylesheet into more readable content with a formatter tool. I googled http://www.cleancss.com/css-beautify/ – and it works fine.
2. Run PD4ML with debug switched on. Now it reports CSS errors with a reference to particular string number of the stylesheet.
2. Locate and comment unsupported directives out or fix syntax errors.
In the most recent version (v398fx4) we suppressed a lot of warning messages, caused by unimplemented popular media control functions (which are usually irrelevant by PDF generation). So check out the version – it produces less warnings on your HTML/CSS.