There are many places from which fonts can be changed. Essentially each view can have it's own font settings.
Start with the basic: Edit > Preferences > Layout (1) general. Then go to Layout (2).
It looks like your screenshot is Albums and Track view. Click on "Album and Tracks" in the top-left of the main panel and then "Customize Panel." Again, each view (Artists, Albums, Tracks, Jukebox) can be set with its own font and can be changed by using the Album and Tracks example.