The PDF output generated via the DITA Open Toolkit (used also by Ixiasoft) is not related in any way to CSS styling. The DITA content gets translated to an XSL-FO format which then gets processed to PDF using a PDF processor like the default Apache FOP. So you need access to the XSLT code on the server side in order to provide any PDF customization. Usually a PDF customization without directly modifying the XSLT code from the DITA OT is done like this:
http://www.oxygenxml.com/doc/ug-oxygen/#topics/dita_pdf_output_customization.html
Regards, Radu