There are in CCSR theme some translatable strings in mobile.css.
Like : body#opac-detail #holdingst td.itype:before {content: "Item type : ";}
But this text is not present in PO file.
Are in fact the css files translatable with PO files ?
To my knowledge, not at present. There are several options available to us:
[1] Remove displayable strings from the CSS files.
[2] Teach the translation tools how to grab content strings from CSS files and apply PO files to them. If we go down that route, I think it might be simplest to put CSS rules containing displayable strings into a separate file so one doesn't have to run the translation scripts every time one wants to tweak normal CSS rules.
[3] Convert the CSS files into templates that are processed by Template Toolkit and which can use a plugin to grab translations from the database. I'm not sure how well that approach would play with Bootstrap and using LESS, however.