I'm running Debian 9with Xfce 4. Window titles look like this after installing a bunch of packages.
How do I change it? Going to Settings > Appearance lets me change system font, but that does not change window title font.
Xfce has a separate settings for window title font.
In Xfce, go to Settings > Windows Manager.
The placement of font settings at different location is the norm in Xfce, so user will have to get used to this oddness. Perhaps this actually hints that Xfce prefers to be modular.