Hi,
I solved the riddle with the help of this blog:
How to add font-icons in Mendix 9 | by Jason Teunissen | Mendix Community | Medium
Keep in mind, that you need to take path from the deployment folder, so in my case:
Dear Tjark Radewaldt,
There is a mistake, you have to consider from root directory when importing. In your case it will be theme/public/fonts/open-sans/open-sans.css not ../public/fonts/open-sans/open-sans.css
Hope my answer helps, reach me out for any queries.