Similar to the scrollbars setting, the Interface Font setting does not seem to work properly.
This setting has been fixed! Toolkit users now have the option to choose from seven (7) different fonts. Below is a gallery of screenshots displaying all of the font options currently available.
I personally don’t find any of them to be an improvement over the default font, but that’s me. Perhaps you like Roboto Condensed better. Or the unique serif font, Inconsolota. Personally, I’d really rather see an option to increase the font size as my eyes are getting old and cranky and don’t see as well as they used to.