You may find here how to operate with Protean (or other non-Ocean templates) Styles https://github.com/unaio/una/wiki/Protean . The keyword is "mix".

If you need to have the to work with Ocean then you may add the necessary "global" CSS file via Studio->Designer->Injections area, "Head injection". Or add the content of your CSS file to the one file of this template in modules/boonex/ocean/data/template/system/css/general.css.