Unable to use Google Font

I am working on the stylesheet for OCS 2.3, and I can’t figure out a way to use Google Font. I tried what they suggest:
font-family: ‘Ubuntu’, sans-serif;
but seems to be the page keeps insisting that “Ubuntu” font doesn’t exist and it picks sans-serif instead. Any idea?

I tried other font family from Google Font too, but it just doesn’t work!