Comment 8 for bug 252640

Workaround:

From a terminal window, edit the stylesheet file for the Ubuntu theme in Gwibber:
gksudo gedit /usr/share/gwibber/ui/themes/ubuntu/main.css

Edit your file to add font size and family information, such as:

* {
  color: black;
  font-size: 8pt;
  font-family: Liberation Sans;
}

.imgbox {
  -webkit-border-radius: 5px;
}

You will need to exit Gwibber and restart it to see the changes.

To see what fonts you have available and their corresponding family names, install the gnome-specimen package and then go to Applications > Graphics > Specimen Font Previewer.