The Fontello font is for the icons at the control panel (or something similar)
I just wish that the urls for those fonts did not use an url with
++unique++2017-08-01T16:40:53.385055
( since I can not see that this font will change at all )
PS: My wording was maybe wrong: the fonts does not really 'disappear from a theme', they are part of the 'logged in uses stuff that comes with barceloneta', so in reality this is a registry build issue (?)
In this case, the referred image is requested from its css as url('++resource++plone.app.multilingual.images/gtranslate.png'). However, since the css is called by a js in the logged-in bundle the context becomes ++plone++production/++unique++/... and this causes the error.
My quick and dirty workaround is via a redirect rule in Nginx.