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?
Please see below link for more information about DITA on AEM. It has the details about publishing the content.