How to specify custom CSS file for HTML5 output of DITA

AEM Sites output has options to specify custom templates as well as clientlibs when generating output from DITA. However, I don't find an option to specify the same with the HTML5 output.

As per DITA OT documentation there is an option to specify a property file with arguments such as CSS root, copy css etc. but the AEM documentation for XML add-on doesn't expose those properties?