I've just spent an hour trying to work that out, and when I finally give up and go link-riding, I find that hereIf you choose to create your own (...CSS file...) which is recommended so the default can stay rather "vanilla", then you can upload it to phpwcms_templates/inc_css/ and it will show up in the template css dropdown for which css file to use.
nice one Ionrock...
Coos