I have the same issue (just upgraded to 1.7.8.0 from 1.7.4, with the one-click upgrade plugin).
I compared the classic theme folder on my production site and the one in https://www.prestashop.com/en/system/files/ps_releases/prestashop_1.7.8.0.zip?token=99bf665546. They are identical (apart from some timestamps)
The problem disappeared after a hard refresh of the page (CTRL+SHIFT+F5).
My guess: the css file referring to the 'new' resources (fonts etc...) is a _static_ file, so is cached by the browser: the browser used the theme.css from my 1.7.4 installation, and the server did not respond properly that this file was no longer valid.