You can dynamically load Stylesheets in either an MCO or in a XAL file using the DisplayService class.
To dynamically load a stylesheet, add the following line to an:
XAL File
<mco:execute xmlns:mco="http://openxal.org/core/mco">DisplayService.loadStyleSheet("newstylesheet.xss")</mco:execute>
MCO
clientSession.getDisplayService().loadStyleSheet("newstylesheet.xss");